Agreed with others, what's helped me most aren't books, but listening and watching. Out in Maui earlier this year I was talking to one of the guys from the Slack Key Show and he told he the Hawaiian way is to really focus more on learning by watching, and I think that's been really helpful personally.
On Spotify I listen to a lot of Herb Ohta Jr, Troy Fernandez, and Led Kaapana. All have different styles and cool things you can learn from. Same goes for YouTube, all have some really fun moves to pick up on when you watch them play. Herb's style is great for learning chord melody, classic Hawaiian tunes, classic vamps, classic chord sequences beyond C G7 F, things like that. Troy is great for learning a real fast picking rolling style and I am learning great ways to double up on notes and pick in cool patterns. Watching Led has helped me see new ways to use thirds and sixths and slides and use the third and fourth strings for runs, not just the first.
Agreed on some of the other great music suggestions above too, Hui Ohana and Ka'au Crater Boys are awesome. Really generally just listening to more Hawaiian music, even slack key guitar with no ukulele like Sonny Lim or Led or Kimo West or others, has been really helpful.