Adverb? Preposition? Particle/Delimiter? Is there a recommended book where I can look up such questions? I couldn't find it in the New Penguin Russian Course book.
It can play a role of either adverb (наречие) - in most of the cases - or, sometimes, conjunction ("союз") pretty much like it counterparts "поэтому", "потому" etc. You can find this information in any decent Russian dictionary.