There are two books which I reference for improv advice when needed: Viola Spolin's Improvisation for the Theater
and Charna Halpern's Truth in Comedy.
I just finished a reread of Truth in Comedy: The Manual of Improvisation
and again found it a really insightful look at the improvisation process. For improv players struggling to find their sweet spot onstage, it is a great refresher. Each chapter can stand alone, with take-away points neatly bulleted at the end. (Hint: read the bullets at the end first before the chapter to know what to focus upon in the chapter.) Halpern's description of the Harold and the development of the Harold technique is the best around.
Spolin's book is an essential tome for any improv actor, and should be considered from within its proper context. Note first that Spolin was working with children and the games that are presented are most appropriate for child players and audience. Second, many of her games are simply frameworks, and work best for the stage in combination with one another.