One thing you have to remember with the numbered novels is that volumes 1 to 15 are all set during the Kes era, while 16 to 21 are from the 4th and 5th seasons.
Some of the better numbered novels are:
Violations (Voyager's computer core gets hijacked, a lot better than the da Vinci episode)
The Black Shore
Death Of A Neutron Star
19 to 21 (sorry, it's a trilogy, but I can't recall the name, but it's a sequel to the Season 1 episode "Eye Of The Needle")
Flashback (goes into a lot moe depth than the episode, when I first read the book, I had not seen the episode, so I thought the episode was a three-parter just based on the book alone)