The G&D books cover a lot of territory. Each section begins with a short design history followed by an overview of the design, broken down into categories (armament, protection, propulsion plant, etc). When it comes to understanding a ship's armor scheme, there's no substitute for seeing a series of drawings; stats-in-a-vacuum like those in a Conway's are less than useless, and a token cross section gives only a glimpse. G&D provide a fully realized picture. The chapter on the Bismarck class is blemished by a lack of definitive information; only now is the extent of the 80mm weather deck generally becoming known, and G&D label it as 50mm throughout. In fact, plenty of questions remain on Bismarck and Tirpitz and their differences, so it's hard to fault G&D for this.
Another weakness is the reference to gun penetration data based on USN calculations; the formula for these calculations was actually intended to predict performance against homogenous armor, and its most common application has been for face-hardened armor. Thus they serve to be indicative only in the most general sense--in other words, why bother?
I find it easier to pick out the nits than to try to elaborate on the battery of qualities in the three books. They sit on the shelf within easy reach, and I refer to them regularly.