Improvising on guitar has long been a challenge of mine. Chord Tone Soloing by Brett Tagliarino has helped a lot!
I have played guitar for several decades, but I never really understood how a guitar worked. Chord Tone Soloing helps explain the music theory behind improvising solos. Adding riffs and melody lines to chords, becomes quite easy, as you work through quizzes and exercises.
The CD book of examples is easy to work with, and play along to. You'll need a good foundation in guitar skills, and chord building, but once you are past the basics, Tagliarino will give you the tools to start working on your own!
I still need a metronome to keep a beat.