The Magic Treehouse fact finders are a hit with my six year old DD (they are rated as a higher reading level than the fiction books themselves, but she was able to read them in K and I didn't think they seemd at a fourth or fifth grade level), she also likes Ivy and Bean, Animal Ark, Fairy Realm (my DD never liked girl things, but suddenly likes some fairy books)and many non fiction books on any topic that intests here (often animals or science).
I am always looking for suggestions too. I'd love to find books with more challenging vocabulary and sentence structure, with interesting non fiction tidbits sprinkled in so she learns something, but with content that appeals to a first grader. It looks like you are in the same boat.
