Wiktionary: Concordances
From open-dictionary.com - the free dictionary.
This page organizes vocabulary lists based on works of literature. Someone reading the stories of Sherlock Holmes, for example, may need to look up rare and obsolete words even if he is a native speaker; someone reading Shakespeare certainly would!
Someone reading the works of Doctor Seuss as an aid to learning English (or just pronunciation of English) may find it handy to have a vocabulary list.