yeah they were filler books. I think there's a stretch of books that are unrealistic, have little effect on the plot, and are almost a waste of paper. They are still entertaining and funny, but I just have a hard time getting over how unrealistic they are. For example in book 28, where at the end Visser 3 gives up too easily and the animorphs somehow manage to escape on foot, followed by a bunch of humans. So what if the visser was locked behind a door, couldn't he phone in a bug fighter to blast them when they came out the other side of the building?
If you notice, there are some books that are just better written and the things the yeerks do to keep the animorphs from getting away are more realistic, ie that they would stop basically at nothing to keep them from getting away.