If you enjoyed Keith Houston's New Yorker blog post about punctuation history, or follow his own blog at Shady Characters, I heartily recommend his new book, Shady Characters: The Secret Life of Punctuation, Symbols, & Other Typographical Marks (forthcoming from W.W. Norton). It's a well-researched and very engagingly-written book filled with fascinating little details about pilcrows and dashes and interrobangs and manicules and other such delightful marks. Houston explores the origins and evolution of these marks, how their uses have changed over time, and dredges up all manner of excellent stories about them.
While the origins of some of these marks are disputed or unknown, Houston thoroughly explores the various options for each, and offers well-reasoned thoughts of his own where there is doubt. And there are more than sixty pages of endnotes, probably more than enough to sate the appetite of even the most fanatic of octothorpe fans.