'Thinking In Jazz' is a fascinating study of what jazz musicians actually do when they're improvising; how they prepare for it, what it's like at the time, what factors affect it. Berliner put it together the hard way, by interviewing and hanging out with numerous musicians over what seems to have been a fifteen-odd year process, including players as illustrious as Lee Konitz, George Duvivier, Doc Cheatham and Barry Harris. There's a wealth of musical examples but, so as not to scare off readers who can't read music, the bulk of them are stowed at the back of the book. (You don't need to be able to read music to find this book valuable, but it helps.)
Berliner's style is a bit stiff and formal (visions of this earnest classically-trained ethnomusicologist studying jazz trumpet so as to get to know his subject better), in sharp contrast to the more conversational manner of his interviewees. But he has allowed the musicians' own opinions to remain as contradictory and unresolved as real life, rather than trying to come to formal conclusions about the 'nature' of jazz; for example, Lou Donaldson is as scathing about rhythm players who want to improvise behind him, as other guys are about rhythm players who refuse to improvise. (You never know what you're expected to do until you're told off for not doing it.)
Don't be put off by the formidable bulk of the book; it's a good read, as well as containing a wealth of stories about the working lives of jazz players.