I'm so with you on this one. My DD is going into 1st grade and loves to read. We went to the library Monday and checked out 13 books- at least 5 or 6 were chapter books. She had read them all by the next day. It's not that she reads all the time either, just fast! She loves several of the series that have been mentioned- Animal Ark by Ben Baglio (there are tons of these to choose from), Magic Tree House, Pony Pals, A to Z Mysteries. Her teacher read Little House in the Big Woods to her class this year, so she's interested in finishing those books. E.B White's Stuart Little and Trumpet of the Swan, and George Selden's books like Cricket in Times Square were favorites of mine as a kid. Also Encyclopedia Brown and Mrs. Piggle Wiggle books. She likes the Magic School Bus books, too, and we've started checking out more non fiction books about animals lately.
In addition to the library, the thrift store, yard sales, and the Scholastic warehouse sales are good places to find cheap books. Congratulations on your new reader!