There is a classical guitarist named Gerald Klickstein who has recently put out a book called The Musician's Way published by Oxford Press. The book is not a technical manual tailored for guitarists but it is perfect for ANY musician. The goal is to teach musicians how to think divergently to solve problems in the practice rooms, how to interpret music, and how to use your body effectively to avoid tension and injury. If you want to get a glimpse of the book before you buy it, Klickstein has set up a website called www.musiciansway.com
that has blog articles (updated regularly), external links, and lots of other resources that will give you a look at what Klickstein is trying to get across. Also, if you buy the book and new studies come to light, the book can't be updated but the website will and, in effect, it means the Musician's Way ideology will never be outdated because it can always evolve via the online resource.
If you want to really step up your game you just want to stop beating your head against the wall when you have problems to solve and it just isn't working, or you could use some inspiration then this book will NOT let you down. If anyone has read this book or seen the website, please let me know what you think about it. I'm wild about what it has to say.