With that out of the way: "Improvising Jazz" is an excellent source of unique perspective, advice, and experience from a noted jazz educator, Jerry Coker. Coker focuses not only on how to play but, on another level, how to learn and how to develop further. You will want this book in your collection to gain further depth and pespective on the subject.
There is advice on how to approach your first jam session, developing the ear, combining intellect with spontaneity. The later chapters on analysis of melody and functional harmony, first written in 1964, were probably the one of the first systematic attempts to "organize" jazz theory. You definitely get the sense that this is a major source material for reference books such as "Modern Jazz Piano" by Brian Waite.
And in the back of the book ...
To avoid paying royalties, Coker prints scores of standard tunes with only the chord changes -- no titles or melodies. It's great fun figuring out what the tunes are. Some of them still have me stumped ...
If you already own Pattern's for Jazz, this book is totally different. "Patterns" is a 99% music whereas "Improvising" is 90% discussion and application with the music used to illustrate the discussion.
My favorite passage is an excerpt the author takes from Richmond Browne on what makes a solo interesting to various listeners. Find this on page 15!
It's a small book to be sure. However, this just means that it can be read over and over and physically kept in your case. Think of it as a "Strunk & White" for jazz.