JESS: Got a lot of books here. Anything in particular?
LORELAI: It’s one of Luke’s.
JESS: Well, if it doesn’t have Encyclopedia Brown in the title, that narrows it down a lot.
Encyclopedia Brown, a series of 29 children’s books written by Donald J. Sobol; the first one was in 1963, and the last in 2012 (posthumous). The books follow the adventures of a boy detective, Leroy Brown, nicknamed “Encyclopedia” for his intelligence and range of knowledge.
Jess’ comments suggests that most of the books Luke has are his Encyclopedia Brown books from his childhood. Note the similarity with Lorelai reading all the Nancy Drew books when she was young.