Those other two books being Dragon and Dumb Bunnies, of which unlike Captain Underpants and Dog Man have been seen in animation prior to what DreamWorks Animation did with them. Dragon was in the form of a stop motion series that sadly featured fewer characters then there was in the books it got adapted from, whilst Dumb Bunnies was in the form of something traditionally animated done I think by Nelvana. There's no reason at all as to why DreamWorks Animation shouldn't do films based off the other two of Dav Pilkey's book series, even though they'd be in different genres. One thing we can expect in them is a different demographic due to them being feature length films as opposed to early childhood shows, and they'd be able to feature characters left out of the respective TV shows those books got given.