Being probably one of the few people on this board who has actually read all 24 of the original Tarzan novels by ERB, here's my capsule review of each of them:
TARZAN OF THE APES
This is, of course, the classic ERB novel. An extremely engaging read from page one till the end, the story of Tarzan's origin and early adventures is an essential read, and full of as much characteriation and ideas as are in any of the master of adventure's books. If you've only seen the Disney film, or Greystoke, check out the real story.
THE RETURN OF TARZAN
I think this sequel to Tarzan of the Apes is my personal favorite Burroughs book ever. The main reason for this is the nonstop action and excitement on every page in this breakneck thriller, with a cliffhanger at the end of every chapter. One of the most thrilling books ever written by anyone. What a film this would make, and what a shame that no one in Hollywood ever capitalized on its potential! The change of location and conflict from chapter to chapter is breathtaking but logical. Highlights are Tarzan's very first encounter with arch-enemy Nikolas Rokoff also his first with the lost city of Opar and its barbarian preistess-queen La, one of Burroughs' most memorable characters. Also contains what has to be the most suspense-filled draw-lots-for-cannibalism scene in all of literature.
THE BEASTS OF TARZAN
This novel concerns the tale of Nikolas Rokoff's kidnapping of Tarzan's infant son and Jane and his plan to strand Tarzan on a deserted island, and how the ape-man eventually escapes from there and takes his revenge. It is a slight comedown from the previous two books, but still has its moments.
THE SON OF TARZAN
A chronicle that follows the adventures of Korak, Tarzan's son, after he is kidnapped by and kills Tarzan's old enemy Paulvitch. He flees into the jungle and discovers the heritage of his father. A decent book, although not quite as engaging as the first two.
TARZAN AND THE JEWELS OF OPAR
Tarzan, after losing his memory, returns to Opar and to La. The first novel to use one of Burrough's oft-times over-used plot devices, that of Tarzan contracting temporary amnesia.
JUNGLE TALES OF TARZAN
Burroughs goes back into the past with this collection of a dozen short stories - to Tarzan's childhood, wherin each story relates a crucial formative experience of the ape-man's savage upbringing. Very similar in style to Kipling's Mongowli stories, well-crafted, and engagingly written. However, the collection is marred somewhat by the inclusion of the story "Tarzan and the Black Boy", without question the most racist thing Burroughs ever wrote.
TARZAN THE UNTAMED
After Tarzan believes Jane to be murdered by German soldiers at the outbreak of WWI, he goes on a personal one-man campaign of revenge against the Huns. This campaign is destined to lead him into one of the strangest lost cities he visits during his career. My personal favorite Tarzan next to Return, possibly the greatest Tarzan novel Burroughs ever wrote.
TARZAN THE TERRIBLE
This worthy sequel to Untamed has the ape-man pursuing Jane and the German officer who is her captor into a lost land deep within a morass in Zaire, where a strange prehensile civilization co-existing with dinosaur life is discovered.
TARZAN AND THE GOLDEN LION
A sequel to Tarzan the Terrible and the third book to visit Opar. The first to make use of another over-used plot device: a Tarzan look-alike. Routine fare for the series.
TARZAN AND THE ANT MEN
Tarzan visits one of the more fantastic civilizations he encounters in his explorations through darkest Africa, a minature kingdom of tiny ant-sized people living in hive-like communities who shrink him to their size. Another action-packed winner, one of the best books in the Tarzan series. After this book, Burroughs effectively writes Jane out of the series (she appears in just one other book, Tarzan's Quest) and the series seems to suffer because of it.
Indeed, “Ant Men” is a demarcation point in the series, from this point on, the novels become increasingly formulaic and repetitive, and lack the zest and innovativeness of the preceding efforts. Of course, there are many good moments still to come, and an afficianado will want to read them all.
TARZAN, LORD OF THE JUNGLE
This is where the Tarzan series begins to settle down to pure formula, although each book is still worth reading to a die-hard fan. Here Tarzan discovers an medival-style community still flourishing in a remote African valley.
TARZAN AND THE LOST EMPIRE
Basically the same storyline as Lord of the Jungle, except this time instead of a surviving medieval settlement, Tarzan discovers a surviving outpost of the Roman Empire.
TARZAN AT THE EARTH’S CORE
This novel actually belongs more properly to the Pellucidar series than the Tarzan series. It follows the events of Tanar of Pellucidar, at the end of which David Innes was captured by the buccaneers of the inner world. This story details Jason Gridley's recruitment of Tarzan to help with an expedition into the polar opening that leads to the inner world in the hope of rescuing him. Tarzan cuts a poor figure in this book, getting lost in Pellucidar early on (sense his sense of direction avails him not in the timeless world) and necessitating that the others lose time in searching for him. One has to ask whether he helped or hindered the expedition more in the end. Early on in the book, the character Von Horst also becomes lost, setting the stage for his adventures in Back to the Stone Age.
TARZAN THE INVINCIBLE
The fourth and last Tarzan book to feature Opar and La. Once again, bad guys try to steal some of Tarzan's gold hoard. Routine fare for Tarzan series.
TARZAN TRIUMPHANT
This time, instead of being a lost civilization of feudal knights or a lost civilization of the Roman Empire, it's a lost civilization of ancient Biblical Midianites. The formula is wearing a little thin, and we also have to suffer one of Burroughs' most annoying and ridiculous characters in Danny "Gunner" Patrick, a wise-cracking gangster. One of the weakest of the Tarzan novels.
TARZAN AND THE CITY OF GOLD
While Burroughs doesn't completely break out of the formulatic rut the Tarzan series has sort of fallen into with this one, he does create one of his most colorful lost cities and characters to inhabit that city in a while. Slightly above average.
TARZAN AND THE LION MAN
Somewhat bizarre and fantastic even for the Tarzan series. It also serves nicely as Burrough's satire on Hollywood and how they trashed his character. This one has the ape-man going after the rescue of a Hollywood company who has encountered disaster in his jungle. He eventually encounters a race of apes with human intelligence bred by a mad scientist. At the end of the book in a delightfully amusing final chapter, Tarzan travels to Hollywood to try to get a part in a movie about himself incognito, but is rejected by the director as being the wrong type!
TARZAN AND THE LEOPARD MEN
Tarzan once again loses his memory and tangles with a strange cult of tribesmen who dress in leopard skins. Far from being among the best in the series, but still has an atmospheric quality that makes it an engaging read for anyone who is not totally jaded by the Tarzan formula.
TARZAN’S QUEST
The first Tarzan since Ant Men to feature Jane, although she doesn't seem to be quite the same character we have known before and could easily be exchanged for any of the heroines in the recent Tarzan novels. A more interesting than usual plot involves a strange hidden african tribe guarding the secret of immortality.
TARZAN AND THE FORBIDDEN CITY
This one was patched together from a series of radio scripts Burroughs wrote. Because of this and the fact that it treads well-familiar ground, it is almost universally regarded as one of the poorest Tarzan books. However, the dramatic nature of it's origin means that it is faced-paced and full of colorful settings, and few can deny that it is action-packed.
TARZAN THE MAGNIFICENT
This novel, a quasi-sequel to City of Gold, hearkens a return to the thrill-packed feel of the early Tarzans. One of the best, if not the best, post Ant-Men efforts.
TARZAN AND THE FOREIGN LEGION
Burroughs attempts to take the Tarzan series in a new direction in this unusual tale (actually the last Tarzan story he ever wrote, but published before “Madman” or “Castaways”). The ape-man joins the army to help allied efforts during WWII and finds himself and his unit stranded in American Samoa, carrying on a running war with the Japanese. Has the feel of an old war-time movie. Unfortunate touches of racism, but such were sentiments during wartime.
TARZAN AND THE MADMAN
Posthumously published and unrevised, this Tarzan is little to write home about. The tired old cliches of the series are all trod out, and only a few decent moments make this book, despite its short length, worth reading.
TARZAN AND THE CASTAWAYS
This novelette, about Tarzan being captured by a zoo safari, whose ship is wrecked on a strange island, has a relatively fresh plot and memorable characters. However, it is not developed fully enough to be as fully enjoyble as the better Tarzan stories. The book form also contains a pair of Tarzan short stories: “Tarzan and the Champion” - Short story about Tarzan encountering a heavyweight champion in his jungle strictly light fare - and “Tarzan and the Jungle Murders” - Burroughs attempt to have Tarzan play at being Sherlock Holmes is less than wholly successful, but a refreshing change of pace nonetheless.