Murder in Black Letter by Poul Anderson

This is a Cock Robin Mystery introducing Trygve Yamamura--judo expert, Samurai sword connoisseur and private detective--triple threat to San Francisco crime. These combined skills enable him to keep his own head attached while finding out who removed someone else's with honorable Japaneses weapon. This is a good old-fashioned detective story, a genuine whodunit, with a good deal of suspense thrown in.