1. A computer-implemented method of generating a custom dictionary for an e-book, comprising: choosing a reading level threshold for a user based on dictionary management information describing the user; identifying a set of candidate words in an e-book for inclusion in a custom dictionary; selecting words for inclusion in the custom dictionary from among the set of candidate words based in part on the reading level threshold; generating a custom dictionary using the selected words and word definition information for each of the selected words; and providing the generated custom dictionary to a user client associated with the user.