Delivering real improvement 

Interim Management by Dennis Russell

The best and most comprehensive book on the subject of Interim Management is the book called Interim Management (The new career choice for senior managers) by Dennis Russell.  Worth every penny and every moment spent reading it.

The 22 Immutable Laws of Marketing by Al Ries & Jack Trout

A very readable book which is full of ideas on how to improve the marketing of your company, whether it is large or small.

The Trusted Advisor by David Maister, Charles Green and Robert Galford

A book for those of us who work as interims, consultants and advisors, it provides very helpful guidance on buidling trust in to our relationships with our clients.

The Seven Cs of Consulting - the definitive guide to the consulting process by Mick Cope

This book is based around a 7Cs framework - Client; Clarify; Create; Change; Confirm; Continue; Close.  The description is very methodical and deals with the hard as well as soft issues of the consultancy process.  Full of analytical techniques appropriate to each stage.