For CS6, the standards are still these:
- The Adobe Photoshop CS6 Book for Digital Photographers (affiliate link), by Scott Kelby (There's a new version (affiliate link) of this book available that also covers Photoshop CC, should you be into that; since I haven't moved to Photoshop CC, I can't really speak to which CC book is best)
- Adobe Photoshop CS6 for Photographers (affiliate link), by Martin Evening
But there are over 500 books that appear in an Amazon search on Photoshop CS6, so it's impossible for me to speak to them all. I'm sure there are some hidden gems in that list I haven't seen, but I have looked and read probably the top dozen most popular choices.
Some additional books that I feel can round out the Photoshop experience that I find useful are:
- Photoshop Lab Color (affiliate link), by Dan Magulis (a seminal work that every photographer should read at some point in their career; another option is his more recent Modern Photoshop Color Workflow [affiliate link], which gives his latest thoughts on moving images through Photoshop for good color correction, but isn't cheap)
- Layers (affiliate link), by Matt Kloskowski
While there's been a lot written about Photoshop over the years, it's a moving target, so some works that were seminal at one point have now dropped off my recommended list, including Real World Image Sharpening and Real World Camera Raw. Even my own techniques have moved on from what you'll find elsewhere on this site (e.g. Technique articles). The real problem with Photoshop has long been how to document a fast moving target if ultimate image quality is your goal. That said, you do need to understand the basics of how the tool works (two books at top of page), and there are some intricacies down below that are worth studying in detail (the bottom two recommended books). From there you can chart your own course, I think.