foreword
Explanation
The Joke
The comic presents two facts: (1) In 1884, Mark Twain wrote the foreword to a French edition of The Adventures of Tom Sawyer, and (2) that foreword is now in the public domain. The comic then draws the "logical" conclusion: for any book you write, no matter the topic, all you have to do is reproduce that public-domain foreword and you can slap "With a Foreword by Mark Twain" on the cover. The example shown is a book titled A Guide to Fellatio — "With a Foreword by Mark Twain."
The Humor
The joke exploits a technicality of copyright and public domain law. While it's technically true that a public-domain foreword could be reproduced in any work, the humor comes from the absurd and misleading implication this creates — making it look like Mark Twain personally endorsed a book on an explicit topic. The comedy is in the gap between the letter of the law (legally permissible reproduction) and the spirit of attribution (implying endorsement). It's a classic SMBC-style joke that takes a real, obscure fact and follows it to its most ridiculous logical endpoint.