Moby-Dick; or, The Whale is a little over 750 pages in most editions.
It begins with 100 pages of accessible, exciting narrative. The first paragraph among the best in any novel I've read.
It ends with 100 pages of equally exciting narrative, right through the final moment.
The middle 500-odd pages is basically whalefacts.tumblr.com. It's a collection of some guy's 1850's medium.com thinkpieces on whales and whale-adjacent topics. But given that it was written 165 years ago, there's a bit more whale phrenology content than is perhaps strictly necessary.