1. A system comprising: one or more processors; and memory, storing instructions that, when executed on the one or more processors, cause the one or more processors to perform acts comprising: determining a score for a word of a plurality of words included in an electronic book, the score indicative of how difficult the word is for a particular user, wherein the determining the score is based at least in part on: a frequency of occurrence of the word in a general corpus of writings of a first language, wherein the score is higher the lower the frequency of occurrence; a contextual importance of the word within at least one of the electronic book or a subject matter area associated with the electronic book, wherein the score is higher the lower the contextual importance; user information about the particular user that includes the particular user's lookup history of definitions of words, wherein the score is lower if the word is included in the lookup history of the particular user; and feedback from the particular user indicative of a level of familiarity of the user with a meaning of the word, wherein the score is lower when the feedback is indicative of familiarity with the meaning of the word; ranking the word among the plurality of words based at least in part on the score; and based at least in part on the ranking, determining a comprehension guide to be presented with the word.