As I said in my review of Volume One of this reproduction by Merchant Books, the Principia Mathematica is a towering classic in the history of mathematical foundations. Volume Two of this Merchant Books reproduction is at the same high standard of printing quality and binding.
Volume 2 contains the second half of the presentation of the theory of cardinal numbers, followed by the theory of relations, and the first half of the treatment of series which is continued in Volume Three.
It is Volume 2 which made me realise that I really needed to get my own copy of the Principia Mathematica for my investigation of the axiom of choice. (This wasn't original research. I was just trying to make sense of the countable axiom of choice for some applications in the topology of sequential compactness.) The particular topic of interest to me was "mediate cardinals", a term which has its origin in Volume 2 on page 288. These are essentially sets which are not finite, but do not have a subset which can be brought into a bijective relation with the set of integers. It was only in 1963 that Cohen proved that the existence of such mediate cardinals cannot be excluded within Zermelo-Fraenkel set theory.
Anyone who finds the Whitehead/Russell Principia Mathematica too difficult to read could try the Rosser book "Logic for Mathematicians", which covers much of the same material, in much the same order, in a more modern and more digestible form. But most mathematical logic and set theory books of the 20th century refer to this monumental Whitehead/Russell 3-volume series. So it's a good idea to have a copy handy for reference.