Wikipedia has this to say about caffè sospeso:
A caffè sospeso or pending coffee is a cup of coffee paid for in advance as an anonymous act of charity. The tradition began in the working-class cafés of Naples, where someone who had experienced good luck would order a sospeso, paying the price of two coffees but receiving and consuming only one. A poor person enquiring later whether there was a sospeso available would then be served a coffee for free.
Buying a coffee for those who can’t afford it or just out of kindness — some coffee drive-thrus will let you pay for the person behind you — but such acts of charity ought not be limited to just coffee! After all, one of the best gifts to give is a book.
Thus libro suspeso! Pending books are those which have been purchased by folks who felt the need to pass on the gift of a good book. Perhaps they read My Dinner with Andrea and loved it, and think someone else might love it to. You can purchase the libro sospeso, and someone else who’s interested will be able to pick a copy up for free! It’s a good way to help out both other readers and authors.