Book 1 contains all the rules - so as well as the moving and fighting mechanics (which, you are right, are rules-lite), you also get submersibles, radar, aircraft, kamikaze units, coastal invasions, coastal craft and defences... lots more...
Book 2 contains scenarios and history, while Book 3 has all the fleet lists.M Book 1 is the smallest of the three, but it is still most substantial!