Volume one will be the first half of the book, volume 2 the second. The edition refers to how many lots have been printed, but sometimes changes are made between editions so if your professor wants you to have the sixth edition there is probably something in it which is different from the fifth.
If you are not reading the whole thing but are reading no more than 10% then you could photocopy (if it is a course book it will probably be in the library) as this doesn't break copywrite (at least in the UK) alternatively you could borrow it from the library once you know which parts you will have to read.