Font Book has been a part of OS X from the beginning. I checked the Limkesoft website and it's true, FontBook was available before OS X; there is a classic version for OS 8.6 through OS 9. So Apple stole the name and added a space. Typical. Still, I don't remember seeing it back then. It was quickly adapted for OS X, the first version being compatible with X 10.1.5. Some of the more expensive font managers provided some type book options, though not nearly so many as FontBook.
That said, if this app provides a service you need, then it really doesn't matter what the name is. And it's reasonably priced.
So I downloaded it to try it out. It takes a little getting used to. It has a wide variety of layout styles to choose from, which might suit someone compiling a type book.
Still, it bears no resemblance to Apple's Font Book, despite the similar name. And as someone else said, it's not a font manager nor does it pretend to be. For that Font Book provides the basics, including font previews, font sets, font integrity checks and the ability to turn fonts on and off.
Note that this app is from a well established developer, Lemke Software, the creator of Graphic Converter, which I have been using since 2001 with OS 9, even if I didn't see FontBook. I'll give FontBook a good rating because it does what it is intended to do and that very well.