"Dreadnought": published 1991 by Ballantine books. 908 pages of text, plus bibliography. Some photographs, mostly of historical personalities.
"Castles of Steel": published 2003 by Ballantine books. 778 pages of text, plus bibliography. Some photographs, about half of historical personalities and half of warships, most of which we've seen elsewhere.
Note: I'm not speaking to technical accuracy here, because I don't consider myself expert enough to comment on the accuracy of the data.
"Dreadnought" is not about the battleship of the same name that we all know and love; we don't even start on her until we're over four hundred pages in to the text. Nor is it about general naval history in the pre-war period, for the Russo-Japanese war barely merits two paragraphs, and I don't recall any discussion of technical developments outside of Britain and Germany at all.
The book is really a primer on pre-WW1 history from the English and German perspective, starting with Victoria and Albert in the early nineteenth century, and ending with the start of the war on 4 August 1914. This includes entire chapters dedicated to the lives of folks such as the Kaiser, Bismarck, and Bulow, and Balfour, Fisher, and Salisbury. By talking about each person's involvement in key events leading up to the war, Massie seeks to explain why those events turned out the way they did. It does give you a pretty good sense of what was going on, but the techinique can be a bit repetitive at times as you go through a given event from the perspective of three or four different people. You may find yourself part way through a chapter asking yourself, "What does this guy have to do with anything?" before the concluding pages of the chapter answer that question.
The major themes covered in this way are Germany's evolution into a naval power under Wilhelm II; Britain's emergence from Splendid Isolation and subsequent rift with Germany; and the "Dreadnought Race", which looks pretty tame compared to Wesworld at times.
If you like politics, it's a good read. If you want to know why Germany and Britain ended up having huge fleets opposing each other, it's a good primer. But it's not about the ships themselves, and depending the historical figure featured in a given chapter, it can be somewhat dry. Still not a bad buy.
"Castles of Steel", on the other hand, is about the major actions of WW1 - so far as Britain and to a lesser extent Germany are concerned. It seems the book is more interested in the surface actions than the submarine warfare story, though there is a fairly decent synopsis of how unrestricted sub warfare brought the USA into the war while the British blockade didn't. Oh - and if Britain wasn't involved in a certain battle, the best you can hope for is a mention that there was a battle. Countries such as France only really come into play when they're working with Britain, such as at Gallipoli.
The naval history is related primarily from the perspective of the senior officers and politicians making the decisions, so there's a fair bit about Churchill, Fisher, Jellicoe, and Beatty, and somewhat less about the German folks. Rather than giving us a chapter on each of these people, as he did in "Dreadnought", Massie instead gives us a chapter each on Jellicoe and Beatty before dealing with the naval war in general chronological order: Goeben, Heligoland Bight, Coronel and the Falklands, the Yarmouth and Scarborough raids, Dogger Bank, the Dardanelles and Gallipoli, unrestricted submarine warfare (and Britain's blockade of Britain), Jutland, and a bit of stuff after that, ending with the High Seas Fleet scuttling at Scapa Flow.
Just by its nature, I found "Castles" far more interesting than "Dreadnought". I came away with two key points - first, that the respected figures of that era - Jellicoe, Hipper, Churchill, Beatty - were flawed, and made mistakes, sometimes big ones. My image of Beatty was particularly hard hit; he comes off sounding like the inspiration for James T. Kirk, and I don't mean that in a good way. The other point is that the outcome of naval battles at this point in history is still quite dependent on environmental considerations like fog or where the sun is. If it can't be seen, it can't be shot.
I'd suggest casual readers will enjoy "Castles" more than "Dreadnought", but both are worth picking up. And if you have the patience for it, it's probably worth reading Dreadnought first.