Revising Prose is a great book to help you learn to write better. This will help you with emails, documentation, writing user journeys, writing use interface copy and more. Highly recommended.
How to Make Sense of Any Mess is a great book when it comes to the fundamentals organizing information. Information Architecture is a critical skill when it comes to design great products.
Any book that deals with human psychology is going to provide valuable information about what motivates people, how they think, why they do what they do, etc. Here are few that I would loosely put in this category and are great books: Influence, Power of Habit, Thanks for the Feedback, Grit, The War of Art, Mindless Eating, and probably lots more that I'm forgetting.
Hopefully those will keep you busy for awhile. =)