The Hobbit

The Hobbit is a real-life book written by and was published in 1937. The Hobbit details the adventures of a hobbit named. The book was referenced in The Ugly Truth and Old School.

Diary of a Wimpy Kid: The Ugly Truth
Grandpa Heffley is seen reading "The Hobbit" as a bedtime story to Greg at 7:30 in the evening.

Diary of a Wimpy Kid: Old School
When Greg is discussing the problems of sleeping in Manny's room, he says one of the reasons is Manny picking out the longest bedtime stories that there are just to annoy Greg with The Hobbit being indirectly mentioned.