the first 3 aren't numbered but they getting to be pretty old).
"Strangers from the Sky" wasn't numbered because it was the second of three TOS "giant novel" paperbacks that, along with "TNG: Metamorphosis" and "TNG: Vendetta", heralded the coming of Pocket's next venture: the first-edition-in-hardcover phenomenon.
Books I've found time to read and reread include "The Galactic Whirlpool", the novelizations of TMP, ST II and ST III, "The Entropy Effect", "Strangers from the Sky", "Final Frontier", "The Final Reflection", "How Much for Just the Planet?", "Uhura's Song", "TNG: Metamorphosis", "NF: Stone and Anvil" and "Andor: Paradigm".
Three I really want to reread, but so many never-read novels keep piling on top: "Prime Directive", "TNG: Immortal Coil" and "Ex Machina".