I have the contemporary guitarist collection

- Theory for the Contemporary Guitarist
- Fretboard Knowledge for the Contemporary Guitarist
- Sight-Reading for the Contemporary Guitarist
- Ear Training for the Contemporary Guitarist

Those books are not expensive (especially if you get your hands on the one that has all those books all in one ... 35$) and very clear.
There's a lot to learn from those books for any level.
I recommend them to anyone who wants quality books that will help them from beginner to advance.