I've read both books and seen both movies.
Get Shorty (the book and movie) are two of the funniest ever. I read Be Cool when it came out and hated it. Absolutely loathed the book. It felt like a different writer and didn't seem like a Chili Palmer story at all.
So, when I saw the movie a few weeks ago, I was hopeful that I would at least find a few redeeming qualities.
I did. The screenwriters updated the story to fit the more current phase of the music industry (girl groups like Destiny's Child as opposed to the book's punk rock singer) and included some more interesting characters. Andre Benjamin and Cedric the Entertainer were great. It still wasn't anywhere near the brilliance of Get Shorty and, in fact, was just a pale sequel. However, having read the book, this was a distinct improvement.
This might be the first time I ever say: skip the book, watch the movie.