Lean UX by Jeff Gothelf & Josh Seiden. Inspired by Lean and Agile development theories, Lean UX lets you focus on the actual experience being designed, rather than deliverables. You’ll learn how to drive the design in short, iterative cycles to assess what works best for the business and the user.
The Lean Startup by Eric Reis – A manifesto on the Lean Startup movement. Learn how to build a product that people want. Because really, the worst thing is to spend months developing something… that nobody wants.
Digging into WordPress – I don’t know a better book out there (and I think I’ve read, reviewed or seen them all by now). Chris Coyer (of the famed CSS Tricks) has really done a great job. I think the PDF for $27 should just suffice unless you’re a hard book type then that is also an option.