£5 on a book

It has been a month already (and a bit more) that I got a five pounds voucher at the local bookshop, as I am a loyal customer. I have yet to spend it. In fact, I did not pay a proper visit to our beloved local bookshop since that day. Which is borderline terrible, as a book lover and someone who loves to encourage local businesses. What it also means is that I still have to spend that voucher on a book. Which is always very exciting and a good excuse as any to go to the bookshop this weekend. Any suggestion as to what I should buy?