Wizardology is a book about wizards, a copy of which was extensively vandalized by Andrew Hussie (and subsequently sent to his friend Tynic as a gift[1]).

An archive of the images Hussie posted of his alterations can be found here, courtesy of bladekindeyewear. The image with which Hussie concluded is the source of the "You're welcome" ongoing gag in Homestuck. The tradition of humorously vandalizing books was continued in Homestuck by Dirk Strider. Dirk sent Jane a personalized copy of Pony Pals: Detective Pony for her birthday.

(Another, more classic work humourously defaced by Hussie is Lowell Thomas's "The First World Flight", as shown here.)