Improvising Made Easy For Guitar, Bass and Ukulele, 2nd Edition shows musicians which notes to play in each major key, so they can instanly solo in any key. I discovered and developed this effective system many years ago to each students to improvise with bending, hammering, pull-of and sliding techniques.

Here's one of the 12 keys pages that gives you all the note options in the key of C. You can read it like a tab and play only those notes together and it'll sound good just like playing all the white keys on a piano. Try it and see. I demonstrate all 12 keys in order here.