Just got a copy today. From flipping through it, it seems much more advanced than Puppet Mania. Puppet Mania has great ideas and projects that I think would work well for parents or teachers making puppets with children. Puppet Planet seems to be for an older age group or even just for adults who want to get a good start in puppet building. I say this because some of the tools used in the different projects are not kid-friendly tools (for example, a Dremel tool).
It is a great book. Well worth the money, especially for new puppet builders. It covers a lot of basics of various methods of building - foam carving, latex casting, patterned sheet foam with fabric covering, etc. - along with some more simple and very creative puppet projects in Puppet Mania fashion.