Agree with Kai - should chapter books feel too overwhelming, there are surprisingly large mountains of picture books at higher reading levels which are faster but still more substantial reads. Most of the ones we liked were one-offs, but in terms of series here's some starters you could find at the library (or in bulk a school book sale/ consignment store):
Magic School Bus picture books pack in an amazing amount of stuff. Note that the same topics come in multiple versions of varying depth and complexity. The chapter books are great too.
Many loathe the Berenstein Bears, but my kids couldn't get enough of them for several years (we must have at least 40 of them picked up for pennies at various book sales).
The Sir Cumference series is loved by both my mathy and non-mathy kid (artsy DD-almost-9 loved them at 4 and still occasionally still asks me to read them to her, and even took them into math class last year to demonstrate some geometry)
Dr. Seuss has lots of wonderful, much longer stories, like The Lorax and Thidwick.
Disney - another easy to loathe but the kids loved - easily has a dozen picture book versions of every movie, ranging from very short and simple to 100 pages of harder - but still small-kid friendly text.
For young chapter books, mine were also big fans of A to Z mysteries and Jigsaw Jones mysteries.
