Thirteenth Night is the first book in the Fool's Guild Mysteries, of which there are currently eight. I'm not quite sure where I stumbled upon this, but I enjoyed it as much as—and possibly even more than—the Smythe and Shakespeare books (for the first of which, q.v.).
This book imagines something of a shadow government run by the fools of the world—Feste included. The plot takes place some years after Shakespeare's Twelfth Night ended. Feste has to head back to investigate a murder . . . committed, perchance, by the vengeful Malvolio.
The plot is intricate but not overwhelmingly complex, and it cleverly worked in a number of characters from Twelfth Night, seen after many years.
And, of course, there's always the possibility that Malvolio is behind all the trouble . . . but you'll have to read it yourself to find out if that's the case.
I will provide you with this sample from late in the book. The "calling card" planted in Feste's room suggests that Malvolio knows who he is and is threatening his life:
The other books in the series involve Feste, but only one—An Antic Disposition—seems to have a fairly-direct Shakespearean connection.
I can't wait to try it.
Bonus image: The way the first photocopy turned out.