Star Trek: Discovery: The Enterprise War by John Jackson Miller
First things first - aside from a few passages involving characters from Discovery, some mentions of Discovery S1's war with the Klingons, and Spock running into the mystery character from Discovery S2, this book has nothing to do with any of the actual plotlines or events from the series. It's a story of Christopher Pike and his Enterprise crew. Nothing more, nothing less.
Second - this book is pretty long. It's about 72 chapters long, including the prologue and epilogue, although most chapters aren't that long on their own. The shorter length of the chapters does a lot to keep the book's pacing from wearing you out, as does the interesting storyline and opponents.
Anyway, the basic plot is this: the Enterprise is in a super dangerous nebula, cutoff from the outside world, except for super low frequency subspace signals (basically space VLF radio). They run into the Boundless, who are a motley group of aliens who impress anybody who travels into the nebula into their army to fight another group of aliens. Naturally, some Enterprise people get captured by the Boundless, so Pike has to rescue them and sort out the whole war thing too.
Since the book is A) barely tied to anything in the actual Discovery show and B) about Pike and his crew, it's actually really good. The Boundless get fleshed out pretty well, since there are Starfleet POV characters in the trenches of the war, and all the characters have reasonable behaviors and motivations. The biggest flaw of the book is that its message is applied hamhandedly - basically, it's that "how you win is as important as winning itself" thing, but extended to military hardware* in a way that makes zero sense. Sadly, that is on brand for Discovery.
*There is some kickass power armor in this novel, and Pike is a fucking idiot for refusing to take the blueprints that were offered in good faith for helping end the war.
