I'd say keep it simple and also make use of the ideas Tim provided. 37 Signals provides a great example: they identified actual problems people had and then wrote simple ebooks with the solutions, all of which I imagine leverage their products in some way.
You should seek to do the same: identify common problems in your area of expertise, and then provide solutions using examples of how you or your firm have helped others implement them in the past. Solutions offered in the book should be synergistic with the services you offer personally.
The book itself should be short and sweet, and easy to consume. The cover blurb should be clear and provide immediate insight into the value that it can provide and to whom it is targeted. Depending on the field, case studies (involving you or your firm, naturally) would be a plus, I'd imagine.
As far as promoting it, I would treat it much the same as a business card; every time you hand out your card, you should also try and hand out the book. Not only will it help you stand out, but it'll also provide you with some leverage to take the initiative and reach out to them. I don't think there'd be any harm in giving them a call or email to ask what they thought in regards to the book, hopefully providing an opening for further discussion.
You may also benefit from following marketing tips for more traditional books as well: