David Baker's series titled "How To Play Bebop" helped me quite a bit early on.
Volume 1 - Bebop Scales
Volume 2 - Learning the Bebop Language
Volume 3 - Techniques for Learning and Using Bebop Tunes
Recently, I've been transcribing (or finding the existing transcriptions) of lines that I really like and extracting the concepts from the lines.
For example, I might hear Sonny Stitt play a double-time phrase on a C Major chord and realize that he's outlining one chord per beat for a bar and then resolving back to the original major chord:
C Major, A7b9 (or C# Diminished, etc), D-, C Diminished (or Eb Diminished, etc), C Major.
From there, I'll decide on exactly how I want to think about this concept. I might break this down into multiple concepts. For example, do I want to imagine beat 2 as A7b9 or C# Diminished or something else that fits? Or do I expand this concept to be "any rhythm changes compatible changes over the one-major chord?"
Next, I practice the concept. I spend time making up my own lines that outline those chord substitutions when I encounter a I Major chord. I've been using iRealPro to create little loops to practice with and/or finding songs/play-alongs where I can apply these principles. Then, I move that work into other keys.
The do-it-yourself process is time-consuming, but it's paying off for me currently. Perhaps it's because my practice is based on lines that
I like and am attracted to instead of lines someone else has created or curated in a book. Perhaps it's because I'm being very conscious about what I'm doing and practicing. The counter argument would be to take the lines/concepts that you like from books and focus intently on those.
Another thing that could help is to identify what is most lacking in your playing
that you'd like to improve and focus your attention there. (There are many things lacking in my playing, but I find it helpful to focus on one thing at a time.) For me, I realized that my improvising was lacking over chords that lasted more than 1 measure. If I had chords that changed every two beats, I felt my material over those chords/situations was better than my material over a I (Major) chord that lasted for 2 to 4 bars. Therefore, I started gathering material from other players that I like in the situation that I lacked. IE, Sonny Stitt playing 2 bars over a I Major chord as described above.)
Thanks for this post! Some of the other suggested books have caught my interest and I've put them on my to-read list.