San Marino's tiny size means few books are dedicated solely to the country, but several cover it well in broader context.
Umbria, the Marches and San Marino by Christopher Catling -- A regional guide that covers San Marino alongside the neighboring Italian regions. Useful for planning a trip that combines San Marino with the Adriatic coast and the Italian hinterland.
The Most Serene Republic of San Marino by various authors -- A concise overview of San Marino's history and institutions, available at bookshops in the historic center. It explains how this tiny republic has maintained its independence for over 1,700 years.
For general reading about the Italian Adriatic region, any good guide to Emilia-Romagna or the Marche will cover Rimini and the surrounding area, which provides the practical context for visiting San Marino.