Should I cite a paper that I haven't read if I use a theorem in it? In my Master's thesis I use a theorem that is recent enough that I could find the article where it is proved, but I've actually learned the theorem and its proof in two books by other authors. Is it necessary to cite the original article, knowing that I haven't actually read it?
It depends how well-known and wide-spread the result is. If by now it is a very well-known  result and the publication already happened quite some time ago, it may not be necessary to quote the original paper.
In doubt I'd quote the article and the books, or at least one of the books. A possible formulation could be: 


  The result below is due to Paul [reference to article]; we recall it in the form given in a monograph by Miller [reference to book]. 


I do not think it is necessary to have read the original paper to be entitled to write this. If it is not too much work to track it down, it could still make sense to at least take a peek at the article you quote.