The fatal folio / Elizabeth Penney.

Enhanced descriptions from Syndetics:

In the third in Elizabeth Penney's delightful Cambridge Bookshop series, The Fatal Folio , Molly Kimball is learning that every killer has a story...

After moving to Cambridge, England, Molly Kimball has found a lot to love, including--of course--her family's ancestral bookshop, Thomas Marlowe--Manuscripts and Folios. And though she's not quite ready to use the "L" word when it comes to her boyfriend Kieran, she's definitely fallen for his intimidating family's library.

His family is paying her handsomely for an updated catalog when Molly discovers the original manuscript of a Gothic novel, A Fatal Folio by the pseudonymous Selwyn Scott. Kieran's cousin Oliver, a professor specializing in Gothic literature, is eager to publish a paper on the mystery--especially because a troublesome student, Thad, is threatening to file a complaint against him and prevent his long-awaited promotion.

On Guy Fawkes Night, Molly, Kieran, and her friends set out to enjoy the costumes, fireworks, and fun--at least until a stray firework starts a panic, and the group stumbles upon a prone body, their face covered by a mask. It's Thad, and he's been stabbed to death.

It soon becomes clear Oliver isn't the only one with a motive, and Molly must once again put on a few masks of her own to sleuth out Thad's killer, prove Oliver's innocence, and discover what Selwyn's novel might have to do with this most atmospheric mystery...