Is the fire of purgatory literal, and are there trustworthy books about it?

Be careful. Scripture does not say much about purgatory, and no one has been there to give us a blow-by-blow account; if someone claims to have been to purgatory, do not believe them, because once they convince you of that, they will lead you on to other things and you will end up deceived. You do not need that, and it is not worth the risk. There are a couple of books on purgatory, but I would be cautious. As for literal fire, probably not; we do not even know what kind of body we will have or whether fire would affect it. Fire is used in Scripture as imagery for something painful, something that will burn away, but whether there is literal fire in purgatory, I would say probably not. That is just my opinion. What exactly God has in store to adjudicate our venial sins, I honestly do not know, and anyone can speculate, but proving it is another matter.