See my review on Roskill's Volume 1. Used copy for £24 sounds not too bad. Though Roskill could not include "Ultra" and some of German records, and tells us little about such minor campaigns as Coastal Forces v E-Boats, he is excellent on the big ships, the major actions on the oceans, and of course the Battle of the Atlantic. The reader will appreciate that he was a key staff officer in the war, knew many of the admirals, and had access to then secret Admiralty files. No doubt he had his team of reseachers, but his history is still masterfully and lucidly written. It is worth about 3 of many books written by young authors in recent years, who were not born before Pound, Tovey, Cunnigham, Norton, Raeder, Iachino, Nagumo etc died. Not just an old codger comment; his text has a "feel" which theirs don't.
Given Roskill was actually head of Naval intelligence when he wrote the book its hard to claim that he did not have access to all the official records. He is VERY careful in not revealing his sources - he was clearly well informed about the operations of Bletchly Park - and was careful about not revealing that information.
Superb book or rather series of books, without equal, not just as a detailed and informed history, but also because of its literary style.