According to the founder of online bookseller Alibris.com, an
average-sized book weighs 12 ounces.
"We analyzed all the books sold by our booksellers and determined that
the average book weighs 12 ounces and over 75% weighed under 1 pound
and eight ounces."
INTERVIEW WITH RICHARD WEATHERFORD, FOUNDER, AND MARTY MANLEY,
PRESIDENT AND CEO, OF ALIBRIS, hosted by the Independent Online
If you prefer the metric system, 12 ounces is equivalent to about 340
grams since there are approximately 28.3 grams per ounce.
search strategy: "average book weighs"
I hope this helps.