Hi! Michael
Go to the UK copperclub site. You will find plenty about pipe sizing and loads of other useful info about Plumbing and its free. Look under "Installation tips" you can then download them as PDF files and print them out if you want.
If they seem complicated, just Google a the main heading and you usually get loads more info which you can look through to find a way that helps you understand.
Buying a hard copy today is okay if you want a reference book, but it depends whether you can understand the information as its written.
If you search around you might find easier ways you can understand it. I suppose we can tend to think, we are a bit "Thick" if we can't understand things.
But people learn in different ways and often the same thing is explained in different ways, its just a matter of matching the two up.
Good Fortune