Where to start with books?

I've had a few pop into my free audible listening but the only one I caught was Watcher in the Rain. Been a lore lurker for a while, I primarily listen to audiobooks and consume quite a few. What recommendations does the community have to give a lore crash course? Was thinking of starting with The Horus Heresy, but is there some place better to start? Any I should give a pass to? Best I should still read, though they're mostly inconsequential? There's like 150 books.