I definitely admire the breadth of Spinelli's writing. It's fun to find an author whose next book is not something that you would expect.
For my lower readers, I have his Tooter Pepperday, which is good for fans of Junie B. Jones, and apparently a series. (Girl does not want to move to farm with aunt so father can write full time, but is won over eventually.) The Bathwater Gang is rather Cleary-esque (neighborhood full of kids gotten together by spunky grandma). I don't feel qualified to comment on either, since they were emergent reader "chapter books". I never like these. In first grade, I checked out Carolyn Haywood and Donald Sobel and have never looked back.
Loser was much better than I anticipated. Introspective navel-gazing, to be sure, but very well done, and a very understanding view of a student who struggles and doesn't fit in. This would be very effective for a classroom discussion.
Check out Mr. Spinelli's web site, which is very flashy and fun, to see more about his books. Again, he has such a wide range. Aside from Stargirl, I particularly liked Milkweed (about a boy surviving during the holocaust) and The Library Card. I found Wringer disturbing, but some of my reluctant readers adore it. I would like to read Jason and Marceline (sequel to Space Station 7th Grade) but it's out of print!