I'm going to say two here - 1_ Save The Cat writes a novel as it explained the structure of a novel - how to create that arc - in a way that made actionable sense. Then 2 - Bird by Bird because writing a novel can be a strange and difficult thing to do and this book helps survive that journey.

@Juno Baker I've heard others rate The Storr book - I'll check that one out. The Art of Fiction was a book I had to read on my MA in Creative Writing. That was yonks ago - I should probably give it a re-read.