The TNG Technical Manual is practically a necessity.
I have most of the Discovery and Picard tie-ins. They’re generally pretty good - the one I wouldn’t necessarily recommend is “The Way to the Stars”, which is about Tilly when she was sixteen. I wouldn’t say it’s bad, but it reads like a book for teenage girls, and I am…not that. My favourite of the bunch is probably the Picard prequel “The Last Best Hope”, which I really enjoyed.
In terms of older stuff, I have a good chunk of the New Frontier series, but fell away from it over time. The four-part Dominion War series from the 90s is interesting, but I would only recommend the TNG instalments, as the DS9 ones are just novelizations of a few episodes.
I also remember owning and reading "Best Destiny, but I really don’t remember it.