The books weren't originally books, but were stories printed in Strand magazine and dramatically increased the sales of the magazine as a result. These stories were published in different years under broad categories such as The Adventures of Sherlock Holmes or The Memoirs of Sherlock Holmes, so the "The Adventure of the Crooked Man" was published as The Memoirs of Sherlock Holmes (the second series of these) and it was the 8th adventure in that series or 2 8.