This item is found in the Elona+ mod of Elona. Mysterious diaries can be made using a pot for fusion or a pot of alchemist. Recipe is available by default. The recipe requires any summoning diary (including Mysterious Diary itself), and another book. The book can include another diary, red or blue books, but not spellbooks. The ability requirement to make it is 10 in literacy. The time required is listed as 10 turns and 1 hour.
When read, a random monster will appear. However, the monster will be neutral to you unless provoked. It can be captured using standard methods. The item is consumed after being read. It is the only known way to get the Fox brother to appear.
The book is located at 429 (Row 13, Column 0) by the character sprite zero-based numbering system. It has no tint, and so looks like an ancient book.