I only recently started appritiating this option, I though to share it with those less knowing.
You are searching for, let's say you are searching for an ebook, now - we know that ebooks can be in .txt .doc .rtf .pdb .pdf .htm .lit formats - right?
Let's say you are only interested in .lit .pdf - what do you do?
Well, you can only write one thing in the "extension" field, so here comes the Boolean Search for the rescue, in the search field, you type:
Book Name AND (lit OR pdf).
Naturally, it also might find somehting like this:
Book_Name(lit).rar
Since lit is indeed in the file name... Maybe you could lower the searches a bit more with (.lit OR .pdf) - hmmmmm... Interesting idea...
Anyway, hope this helps.