Field Guide is a book that only appears in the episode "Tea at the Treedome." It is known to contain an image of a "Land Squirrel." SpongeBob uses that book to look up Sandy in order to find out what she is.
The Field Guide appears to be a blue book with the picture of an anchor on it. The words "Field Guide" is written at the top in white letters.
Not much is known about the book, except that it contains information about certain animal species. SpongeBob may have gotten this book from his library at his house.