I can definitively state that this is neither unintentional, nor is it grammatically incorrect.
The question of whether or not the book should be more properly titled the Player's Manual or the Players' Manual was actually brought up by both the editor and my wife. As author and publisher, I made the executive decision to leave the title as-is for two reasons.
As to the second, the apostrophe is used in two cases; singular possessive (Player's) and plural possessive (Players'). In the case of the present work, neither applies.
It would apply if the intent of the title were to imply possession of the manual by either a specific player or all players in general. However, that is not the case. The manual is intended for the players. Thus, the term "Players Manual" is the equivalent of saying "Manual for Players". In which case, the word "players" is the object of the (implied) preposition "for", and thus no apostrophe is needed.