Of course, these two volumes are an essential source of reference for many kinds of scholar - but if you are intending to buy secondhand, take note of my experience.
Five times I ordered these books, each time from a different seller. Four times only Vol.1 arrived. None of these could send me Vol.2 and so refunded the money - one even sent me a second copy of Vol.1 in compensation!
It was fifth time lucky. At last I got the complete set - and the local charity shop is wondering what to do with so many copies of Vol.1