For some children, who love to read anything they can get their hands on about mythological creatures, this will be fun -- mainly because of the design and all the interactive doohickeys it comes with, not because of the content. There are many books out there on this topic with a lot more information, but for beginners, this could be an attractive way to start. The letter you can take out and read; the miniature booklets; the samples of fake monster skin, hair, and feathers; and an ending cabinet full of samples all make this a very appealing object. For kids who are not myth-crazed, though, the appeal will wear off rapidly, and they will find the content somewhat dull.
The publisher recommends this for children ages 6 and up, but kids at the lower end of this age range may find it confusing -- it mixes together fantasy and fact, and presents it all as nonfiction. Even older kids may have difficulty knowing what is real and what isn't. The whole thing, of course, is intended as a joke, but many kids don't read it that way, and some adult assistance may be required.