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>Question: Does anybody know what the GPO Manual is?
It's "A Manual of Style : A Guide to the Basics of Good Writing," by the U.S.Government Printing Office.
I see it on remainder tables at bookstores every so often. Amazon.com says it is out of print. This may or may not be the truth. They also say they will look for a secondhand copy for you.
Of course, you could check the secondhand shops in your city, or you could try the public library.