Who is The Faeries, the Creatures from The Field Guide and what is their importance?

The Field Guide makes mention of and describes a series of faeries and other creatures. The major faerie character is the boggart, a mischievous faerie that also takes the friendlier form of a brownie. There are pictures of other faeries at the end of the book, and these faeries are expected to appear in the next editions of the book series.