A Ship's Cover is a file that contains all the paperwork that is related to the design of a warship and to its subsequent modification, repairs, battle damage, modernizations etc. If a study is being made of a specific class of ship, consulting the Ship's Cover is the primary source of reference for how and why that ship ended up the way she did.
Typically, the Ship's Cover will start with the specification issued to the design office. Then there will be whole strings of outline designs exploring varying options. There will be memos from the engineering staff and the gun designers on what options they can offer, the minutes of meetings in which varying aspects of the design are discussed, ideas proposed and rejected, the merits of varying design compromises evaluated. The whole design process is carefully documented and (eventually) the reasons why the final design selected was favored are summarized. Readers won't find such things as detailed blueprints (although there will be detailed general arrangement drawings) nor will they usually find accounts of gun design etc. There will also be less formal documents like details of wagers between members of the design staff as to whether (for example) a ship will meet her design speed on trials and the outcome of those wagers. Finally, the results of the ship's trials and comments from her first crew are included.
Obviously, this is a pretty voluminous document but a complete Ship's Cover is an incredibly valuable resource. The Covers on a lot of the 1920s and 1930s British ships are substantially complete so readers know exactly why they were designed the way they were.
Sadly, not all covers are this complete and some are missing completely
(for example the Cover on the British E-class cruisers was destroyed by
bombing in 1941 and was restarted in 1942 so we know very little about
why those ships were designed or the various options open to their design
staff - most of what is "accepted" in this area is hypothetical). In the
US, Ship's Covers are held by the Ship's Characteristics Board (SCB) and
many designs are actually known by the SCB number of their Ship's Cover
(thus the modified Essex class with angles deck and steam catapults are
known as SCB-27C).