The usual Cracked "language!" caveats apply. My favorite: Henry Darger, 'In the Realms of the Unreal.' One I've tried unsuccessfully to read for years: James Joyce's 'Finnegans Wake' (which begins so beautifully: "riverrun, past Eve and Adam's, from swerve of shore to bend of bay....").

The 6 Most Certifiably Insane Acts of Writing (Cracked)

includes entries covering the Darger and Joyce selections above, plus those by Hans Fallada ('The Drinker,' 'In Meinem Fremden Land,' and a book of children's stories); Robert Shields's diary - the length of 500 novels; nine-year-old Daisy Ashford's 'The Young Visiters'; and Jean-Dominique Bauby's autobiography, 'The Diving Bell and the Butterfly.'