The Broken Teaglass by Emily Arsenault.
There's been a lot of discussion about whether or not this book is a mystery. I don't think it really matters how it's classified, but readers looking for a traditionally constructed mystery novel won't find it here.
What they will find is an extremely well-written story about a young man named Billy Webb, recently graduated from university with a degree in philosophy, who takes a position as an editorial assistant at a dictionary publishing house.
One of his tasks is to respond to letters with questions about definitions and word construction. Asked to respond to a letter questioning a response from another young editorial assistant, Mona Minot, he talks to her about details of the original letter. While searching through the citation files for information, they stumble across a citation that looks like a quotation from a book called The Broken Teaglass. Curious about the book, they look it up, only to find that it doesn't exist, and are ready to disregard it when they find another.
Billy and Mona become obsessed with these fragments, and begin to search the citation files for more. This quest draws the two young people into an odd friendship, through which both learn about not only each other but themselves.
As mentioned in the first paragraph, this is an unusually constructed novel, with the unlikely premise of the business of lexicography. Mystery or not, it's a cleverly fashioned novel.