Unfortunately, the book isn't perfect. When the narrative strays from Celtic creatures, the information can turn to hopelessly vague, or in some cases, downright offensive. For example, the author finds no problem with referring to Romani people as "gypsies" and states alleged facts about "the Native American culture," completely ignoring that there are over 500 federally recognized tribes, and they do not share unified practices or legends.
Bottom line, if you want to learn about fairies and mythological heroes without the headache of reading actual Celtic legends, this is your book! If you want something that discusses magical creatures and figures in multiple cultures with just as much authority, perhaps you should look elsewhere.