For picture book/book creators, this post blew me away...thank you John Shelley (www.jshelley.com) for this incredibly helpful post regarding signatures and how books are printed, and why there are certain page counts for picture books.
Number of Pages
Picture books are constructed from signatures.
Pages are laid out and printed together on large sheets, which are cut
in half (giving 4 sides), then half again (8 sides), then folded, making
a bundle of 16 pages called a “signature”. Many picture books consist
of two signatures sewn together, resulting in a 32 page book. It's the
most common format, and as common is generally easier to produce, it's
best to submit dummies with a 32 page plan. However for slightly extra
cost an extra half sheet (8 pages) can be added, making 40 page (2.5
signature) and 24 page (1.5 signature) books. Three signatures would
create a 48 page book, four a 64 pager and so on. Even more confusingly,
on rare occasions sheets are removed too - my last book in Japan, a
version of Jack and the Beanstalk, is an irregular 44 page
separate ended book - that's three signatures with one sheet (4 pages)
replaced by separate endpapers.