I don't know if my answer is fair because I didn't read many Trek novels. But Mosaic, the Janeway biography by Jeri Taylor, was absolutely wonderful. It was such a good backstory for her, so many good details and characters. I'm back working on Voyager stories at the moment, and that book is central to two of them.

(Sadly, Pathways, the book for the rest of the crew's backstories, wasn't nearly as good.)