So you want to be more "creative" with your music while "improvising?"
I'm not sure a structured course or book will help with that. "Structure" seems to be quite the opposite of "improvising" or using creativity.
Just some random ideas that might help.
Try playing some of the songs you typically play using alternate tunings. This would require using different chord shapes and voicings and would lead to a different approach to picking over chord inversions.
Try altering the chord set used to play certain songs in standard tuning, by using a capo in a different position. This will also alter the chord voicings and fingerings and give you a different starting position to develop lead runs and fills based on the chords used. For example, if you normally play a song in the key of A in 1st position using the chord set from A Major - try using a capo on the second fret so that you would use the chords from the key of G Major to play the song in the key of A.
Search Youtube for "covers" of songs you play to see if another musician's interpretation of certain songs inspires you to approach your playing of that song in a different manner or gives you some new ideas.
Use a looper or recording device to record yourself playing the song the way you usually do, then try to play along in a way that adds something different.
If you normally use a flatpick technique on a particular song, try fingerpicking (or vice versa).
Try playing songs in a drastically different tempo - either faster or slower and see if that leads to a different approach to your improvising on that song. If so, you can take your new ideas and apply them to the original tempo.
Get together with some new musicians you don't normally play with and jam. They will likely have some ideas on how to play your old familiar songs that are new to you. Perhaps even jam with some musicians not familiar with the songs you play or artists you cover and have them improvise without any preconceived ideas about how the music is "supposed" to sound.
Hopefully one or more of the foregoing ideas will lead you to a fresh approach and breath new life into tired renditions of the great songs you have been playing for so long.
Good luck and enjoy the journey.