The book includes comprehensive entries on authors, books, shows, and characters. Inevitably, it omits some characters and authors, but it gives an workmanlike account of the chosen subjects. I particularly liked the entries on the pioneers of the mystery format.
As somewhat of a newcomer to mystery fiction, I'd like to have seen entries on the various plot devices and motifs of mystery fiction. I looked in vain for entries on such subjects as "drawing room mystery," "locked room mystery," "fair-play mystery," and numerous others. The work discusses all these subjects in the context of entries on authors, books, and shows, but you have to search to learn about them.
The encyclopedia does have several sidebars (set off in borders) on such subjects as "hard boiled detectives," "dime novels," "Sherlockiana," and the like. The sidebars are well done, but I'd like to have seen more.