This is 'The Rules' hardcover rulebook published by Games Workshop for Warhammer 40,000 (7th Edition). It contains the core rules for playing the tabletop miniature wargame and was released as a standalone companion to the 7th Edition boxed set. The cover features artwork of a Necron character with multiple mechanical arms, consistent with the 7th Edition release era. This appears to be the small-format standalone rules volume, not the full core rulebook.
