In its day, the "Ship-of-the-line" was the equivalent to battleships in the 20th century. The "line" refers to the fact that a standard strategy of naval warfare was to have large ships on the two sides in battle in a line, firing broadsides at one another. Hence, ship-of-the-line.
These ships were the largest fighting ships of their time, divided into four classes. The least of them was still normally more powerful than a frigate, the next ship down in power. One of the best known of these vessels was H.M.S. Victory, which Admiral Horatio Nelson commanded so famously (see a cutaway view of the ship on pages 28-29).
The brief book begins by noting the importance of this class of vessel (Page 3): "Seapower was the key to victory, and the British ship-of-the-line was the ultimate arbiter of maritime supremacy." The book begins by noting the rating system (depending largely on firepower; top level ships-of-the-line had 100+ cannon and three levels of cannon). There is discussion of design of these behemoths, how to build an effective battle fleet, fleet strength, and ship types. Next is an exploration of operation, how ships were organized, how to sail such a ship, battle tactics, and ships in action.
An especially interesting section is a listing of ships-of-the-line on pages 37-41, from the "First rate" ships such as Victory, Britannia, Hibernia and Howe to the "Fourth rate" ships, featuring 50-60 guns. There is also a list of ships captured from French and Spanish navies, among others. The book closes with a bibliography and glossary (Did you know that an "orlop" is the deck below the lowest gun deck?).
If you want a quick introduction to this workhorse of the fleet, this is a good reference.