Book on the method of thought experiments in science.
http://www.amazon.com/dp/3518293427/
(cheeper via amazon.de: http://www.amazon.de/dp/3518293427/)

Free online papers on thought experiments by the same author:
http://philsci-archive.pitt.edu/4700/1/TE_Toronto090522_kuehne.pdf (in English)
http://philsci-archive.pitt.edu/3498/1/UKuehne_1997_BremerPhilosophica.pdf (in German)