There are many books I would like to read in spanish, such as George Jackson's "Soledad Brother"
What is the most efficient way for finding out if a particular book is available in Spanish?
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The problem of that kind of search is that, if you only know the English title, you'd need a super global database that actually maps each book with all its titles in all languages, and I don't really think something like that exists (maybe I'm wrong!).
You can try a large book database like worldcat.org, which allows you to filter by language.
For example, searching for Soledad Brother and filtering by Spanish gives you these results. Note that the first result is what you're looking for, but you get this correct result because fortunately the Spanish title is "Soledad Brother: Cartas de Prisión".
Searching for example for The Lord of the Rings and filtering by Spanish gives you these results. In this case the third result is the good one "El Señor de los Anillos", but you have to be aware that you're getting this result because whoever introduced that book into the DB added a line in the book details section saying: "Translation of: The Lord of the Rings.", so that the DB can match that results.
In summary, it'll be a hard task, probably impossible for rare books, but you can give it a try. Otherwise just google for Some book title + en español or something like that and good luck ;)
Anyway, as I said maybe I'm wrong and there is actually some site to perform that kind of queries...