Keith Johnstone's earlier work, IMPRO, has influenced and will continue to influence the way acting and improvisation for the theatre are taught. IMPRO is a book not only about theatre and improv, but about teaching and human interaction, loaded with insights making that book highly suitable for the general reader.
This follow-up is more specialized: a handbook for putting IMPRO into practice, including detailed improv structures for performance and for rehearsal, and chapters on how to teach these games. Sample run lists and notes from performances impart Johnstone's experiences, trials and errors over many years teaching in several countries. The book is exhaustive and beautifully written, but for the general reader, IMPRO is more appropriate.
One disappointment about the book is some sloppy copy-editing. It is rife with typos, of the sort that are not picked out by a computer spell-checker since the typos form actual words.
The title IMPRO FOR STORYTELLERS is, as Tim Sheppard pointed out below, potentially misleading. This is not a book that will help a solo performer generate material, though some of the exercises within can be translated for that purpose. Johnstone's concern is that improv not be restricted to a form of "light entertainment" (think "Who's Line Is It Anyway?"), but as a way of generating narrative and using it to explore human relationships.