Frank,
It depends on where you are in improvisation. The one book that really turned a light on in my head is "Inside Outside" by "Bunky Green." He's been a professor at a few colleges and other associations. He is an Alto Sax player. There is a trick he teaches and it works and makes you sound like you know a secret. And after using and practicing his method you do know a secret.
The book is relatively simple to understand and a particular point he makes early on is easy to grasp and understand. But no book is going to make you an improviser.
The absolute best way to learn to improve is with Band In A Box. Type in simple blues progressions (not just three chords) some blues charts has 12 or more chords in them. It allows for much more expression than 3 chords. You can start with three chords but someday somewhere you should progress to more complex things unless you’re dead.
Frank an important fact to remember is that in every tune there is actually part of a blues progression some where. It can happen in a few different places depending on if the song changes key and other factors. The most common being the IIm7 chord and the V7 chord.
The two most colorful chords that can be substituted during a solo even if the piano player or the guitar player does not make the substituted changes is the IIm7 (2 minor 7th chord) and the V7 (5 dominant7th chord) which in a blues chart, the IIm7 falls on the 9th bar and the V7 chord falls on the 10th bar . Also, a faster (or shorter form of the IIm7, V7 chord progression) is at the end of the turn-around in the twelfth bar. You can play the same lick in the 12th bar as you played in the 9th and 10th bars as a faster or shortened form of the lick. It is the actual substitutions that Bunky Green explains that can be played over the IIm7, V7 chord progression that opens a whole new world of improvisation for many.
With Band In A Box you can jam all day with a band with unlimited solos that can help your mind to become creative on any style that you like. You can choose for it to create a solo for you on any kind of style or chords and any instrument sound. No! it isn’t as expressive as a human playing, but the licks ( the notes) it does in a solo are amazing and will greatly help you to hear and see how certain notes fit on a particular chord etc. the way the greats did it.
It can print out the solo or you can play along with the bouncing ball on the screen with the solo notation. You can play it as slow as you want and you can play it faster than you can tap your feet after you are familiar with the chord progression.
Frank if you can’t learn to improvise with BIAB and the book by Bunky Green, after a year, forget it! It ain’t gonna happen. Play all day long with the solo feature in BIAB along with your instrument of choice and you can't help but learn to solo. BIAB is the cat's meow for wood shedding and learning your ax.
After a while put in "All The Things You Are". If you learn to solo and not get lost on that tune, you will be able to solo on any song.
If I had BIAB in the 50’s I’d be Dexter Gordon today. I learned more with BIAB about improvisation in 6 months than I learned playing with bands in 20 years in . Anyone who is serious about wanting to improvise needs BIAB. I don’t sell BIAB. Contact (pgmusic.com)
In 50 years every teacher of Jazz will insist that the student get either BIAB or some form of that software.
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I'm not prejudiced, I hate everybody!! Ha ha! My Sister-In-Law had this tee shirt. She was a riot!!!