The joke is that "nothing" is the answer to whatever the title of the book asserts.
A number of such titles have been published as attempts at satire or polemic, to some commercial success.
In 2017, The Guardian commented that the trend of publishing political empty books had led to "the noble art of political parody [descending] into a one-joke turn that avoids words".
[1] This list includes, in order of publication, empty books that have been published with an ISBN or have received coverage in reliable sources unrelated to the author.
The books may have book design features such as front matter, a table of contents, page numbers, etc., as long as the pages are otherwise blank.