Hee. I remember exactly where I bought my copy. I was out with my mother on one of our twice-monthly visits, and I was so happy to find another Star Trek book. I grabbed a copy and was fishing out quarters from my purse (babysitting paid a whopping 50 cents/hour back then and the book cost $1.75). My mother was appalled that I would spend that kind of money on a science fiction book (she never did understand SF or why I liked it; she considered it a huge waste of money because nothing about it was "real"). "You don't really want that, do you?"Spock: Messiah is an awesome read, no matter your fan proclivities, because the crew is so out of character!
Well, yeah, Mom. I do. And yeah, I was kinda young (12-13) to read some of the scenes in that novel. But it's no worse than a lot of fanfic, and the novel is still occupying part of my Star Trek bookshelf... 40 years later.
One of the New Voyages stories, Mind-Sifter, was adapted into a fan film (Phase II/New Voyages).I remember The World of Star Trek as a title, but I'll be darned if I can remember what was in it. Pretty sure I liked it and read it over and over since there wasn't much Trek literature back then, but haven't seen a copy since the 70s.
I also remember Star Trek Lives! from a few years later, roughly the same time as Star Trek: The New Voyages, a collection of early fanfic. A few of the stories were decent but others forgettable and some were just plain bad. Sort of like any given Trek show, I guess.
I blame Star Trek Lives! for the fact that I've got a huge shelf in my apartment that's dedicated to print 'zines - mostly Star Trek from the '60s-'80s, plus some Highlander, Robin of Sherwood, and Doctor Who. There are a couple of infamous ones, namely Spock Enslaved - some sellers on eBay have the price cranked up to $500 USD, although if you shop around you can get it much cheaper. My copy cost less than $25 USD.