Here are some books that I highly recommend. I have not read a lot of books for this age group, so I am sure that there are plenty of other excellent authors who write for this age group. If you know of any good authors, let me know!
Intermediate Grades (4-6)
*Out of the Dust by Karen Hesse (This is listed as 4-8, but it is an excellent novel.)
*Ramona series by Beverly Cleary (It's old, but since there's a movie coming out, a lot of kids will want to read them.)
*Number the Stars by Lois Lowry (ages 9-12) All of Lois Lowry's books are great.
*Allie Finkle series by Meg Cabot (note that other Meg Cabot books are not appropriate for this age)
*On the Run series by Gordon Kormon
*39 Clues series by various authors
*Diary of a Wimpy Kid series by Jeff Kinney
*Out of My Mind by Sharon Draper (note that most of Sharon Draper's other books are for older readers)
*Main Street Series by Ann M. Martin (who is the author of the Babysitters Club books)