One series that has been reliably enjoyed lately is the Froggy series by Jonathan London, illustrated by Frank Remkiewicz. It meets my main criteria, which is that there are many books in the series, taking the edge of the mind-numbing repetition that often ensues after a library trip. Typical of many preschool series, it follow the adventures of a character (in this case a frog) as he experiences the novel (to him) yet familiar (to kids). Summer camp, baby sister, riding a bike: its all there.
Despite the tried-and-true themes, there is something particularly compelling to me about the Froggy character. He's just so darn well-meaning, even as he causes all sorts of trouble. I think my love for him stems for the fact that he reminds me of one of my favorite well-meaning mischief makers - Brianna. Also, there aren't too many characters, so I can handle doing the voices. And, there is something joyful about the part in each story when someone calls to Froggy from afar ("Frrooggyy!") and Froggy, in true kid style, responds with a mildly surprised, "What?"