Monsters Of Myth was a book owned by Greg Heffley who gave it to Rowley Jefferson for him to get some inspiration for the next characters that Roland the Kind will fight in Rowley Jefferson's Awesome Friendly Adventure.
The book is about mythology characters that were in well-known fairy tales and stories being told throughout human history. Rowley says that he could not use the characters in the book because of copyright issues but Greg says that since the creation of the characters are very old, they become free domain to use.