Chess books about Chess Openings

The first moves of a chess game are the opening moves, collectively referred to as the opening. Recognized sequences of opening moves are referred to as openings or defenses, and have been given names such as the Ruy Lopez, Sicilian Defence, and Queen's Gambit Declined. There are dozens of different openings, which vary widely in character from quiet positional play (e.g. the Réti Opening and some lines of the Queen's Gambit Declined) to wild tactical play (e.g. the Latvian Gambit and Two Knights Defence, particularly the Wilkes-Barre Variation).

A sequence of opening moves that is considered standard or follows that given in a reference work (such as the Encyclopedia of Chess Openings) is referred to as 'the book moves' or simply 'book'. These reference works often present these move sequences in theory tables. A new move in the opening is referred to as a 'novelty' or 'theoretical novelty'. At the point at which a game deviates from previously known opening theory, the players are said to be 'out of book'. In some opening lines, the moves considered best for both sides have been worked out to 30-35 moves or more. Serious chess players often spend years studying the openings, and continue doing so throughout their careers, since opening theory is constantly being refined.

from Nunn's Chess Openings.