Books about Liechtenstein in English are vanishingly rare, but a few are worth tracking down.
The History of a Country and its Prince: Liechtenstein by David Beattie. The former British ambassador's affectionate, detailed political and social history is the standard English-language reference, and reads as a labour of love rather than a textbook.
Liechtenstein: A Modern History by Pierre Raton. An older but still useful overview of how the principality survived the wars of the 20th century and reinvented itself as a financial centre.
Adventures in the Smallest Country by Charlie Connelly. A breezy travelogue from the writer who tried to follow Liechtenstein's national football team through a full World Cup qualifying campaign. Funny, light, and the best introduction for anyone curious about why this country exists at all.