Croatia by Piers Letcher (Bradt Travel Guide) — one of the more thorough English-language guides to Croatia, covering history, culture, and practical travel including the Zagreb region and day trips like Samobor.
The Rough Guide to Croatia — reliable general guide with good coverage of Zagreb and the surrounding area, including Samobor and the Žumberak hills.
How We Survived Communism and Even Laughed by Slavenka Drakulić — the Zagreb-born writer's sharp, personal essays on everyday life in socialist Croatia and Yugoslavia. Not a guide book but gives real texture to the region and its recent history.
Croatia: Aspects of Art, Architecture and Cultural Heritage edited by Rose Ash and Martina Petranović — a collection of essays on Croatian cultural heritage, useful background for understanding the Habsburg-era towns of the Zagreb region, including Samobor's 19th-century square and the Žumberak border country.