Mine is Creating Customer Evangelists: http://www.amazon.co.uk/Creating-Customer-Evangelists-Customers-Volunteer/dp/1419597213. Fascinating read on how brands today need more than loyal customers, they need real evangelists that will help companies build and market their brands.

Good question! There are some books that have made a big difference to my life!
My favourite books fall in two categories: self-development (general) and professional (finance, business etc). Self- development books as a real treasure sometimes and can motivate you, get your mind straight and focused, become more efficient, more confident etc.

Best self-development authors / books:

(1) Dale Carnegy: “How to win friends and influence people”; Thi sis a Self-dev 101 work! Written over 60 years ago, wonderful literature and one of the founding books of the popular psychology era and self-improvement essentials!
(2) Tony Robbins: “The Giant within”; Written in a very American way, with powerful suggestions and a lot fun to read; very motivating and grand.
(3) Stephen R. Covey : “The 7 Habits of Highly Effective People”; Well-structured and very useful theories, though easy read
(4) Spencer Johnson: “Who moved my Cheese”; one of the all-time best sellers – still reading myself.
(5) Hirschhausen “Glück” (only in German) Best and funniest book you will ever read, with a lot of wisdom!

Productivity enhancing more task oriented books are:
(1) David Allen: “ Getting Things Done”


In regards to Finance – In my Venture Capital days the following books were super helpful to understand start-ups, structuring, finance etc.

(1) Finance Modelling: Sawyer “Excel Financial Modeling for Technology Startups” (technical); this one will give you a step by step about how to build a financial operational model in a startup.
(2) General VC: Alexander Haislip “Essentials of Venture Capital” (easy read, not too technical, it will give you all you need to know for fundraising for a start-up and also good for a VC to get a more comprehensive picture. Great introduction to the VC world.

For Case leaning, the best ones are from Harward including:
(1) Lerner, Hardymon et al: “Venture Capital and Private Equity: A Casebook”

I guess I can list two books that have broaden the way I see the world and that helped me improve my 'bigger picture' skills.

The first is The Art of Always Being Right by 18th century hardcore German philosopher Arthur Schopenhauer.
(http://www.amazon.co.uk/Art-Always-Being-Right-Argument/dp/1906142246)

The second is Small Is Beautiful: A Study of Economics as if People Mattered by Ernst Friedrich Schumacher.
(http://www.amazon.co.uk/Small-Beautiful-Economics-People-Mattered/dp/0099225611/ref=sr_1_1?s=books&ie=UTF8&qid=1329837518&sr=1-1)

I wouldn't say they are easy, fun or technical reads, but totally worth the effort. Be persistent.

Best books:

Lean Startup - Eric Ries
The 5 temptations of a CEO - Patrick Lencioni
How to win friends and influence people - Dale Carnegie

David Allen: “ Getting Things Done”

It would be hard to choose just one book.
I would say that my career started with
PHP3.5 by New Riders
but a book that REALLY helped me become a better professional on my industry (IT) was

The Clean Coder: A Code of Conduct for Professional Programmers (Robert C. Martin)

The most useful books have been

1. Execution - by Ram Charan - covers the most underrated part of business leadership
2. Maverick by Ricardo Semler - To show how to build a radically different kind of organization
3. The Halo Effect by Phil Rosenzweig - To understand how business research can be flawed and to always question even the big guys talk about

I'll be honest, I'm not much of a book reader. By the time an author sits down and writes a book, gets the book edited, makes it through production and finally gets released the content is usually 12-18 months old. If you live in a fast paced world and are fairly well connected, you would have been exposed to the thinking in the book prior to its publication.

That said, I do buy books and scan them, because the outline of the thoughts in the book are more valuable to me.

I am a minimalist, so I don't like stacks of books cluttering shelves around me, but the few books I have kept include:

The New Strategic Brand Management by Kapferer http://www.amazon.com/New-Strategic-Brand-Management-Sustaining/dp/0749450851/

Wikinomics by Tapscott http://www.amazon.com/Wikinomics-Mass-Collaboration-Changes-Everything/dp/B004J8HXOA

Cognitive Surplus: How Technology Makes Consumers into Collaborators by Shirky http://www.amazon.com/Cognitive-Surplus-Technology-Consumers-Collaborators/dp/0143119583/

Future of Management by Hamel http://www.amazon.com/Future-Management-Gary-Hamel/dp/1422102505/

I agree with Master to some degree, if I'm reading to keep up with the latest thinking then books are too slow. How to books are outdated before they're published, the web is the place to learn. However, books like Fletch's Art of Looking Sideways are timeless. You can't replace the tactile and visual experience an inspiring book creates. The book doesn't command the attention and intense focus that a blog post or inline video demands, leaving the brain to drift above the pages and re-form the ideas into new ones.

IMHO.

The Art of Looking Sideways
http://www.amazon.co.uk/Art-Looking-Sideways-Alan-Fletcher/dp/0714834491

Much of Edward De Bono's work
http://amzn.to/yT266E

How to get Ideas - There's another little yellow and black book from the 1950's I have at home written by an Ad Exec at JWT that is just 50 pages of simply brilliant techniques that lead to great ideas. This link is to a more modern, albeit wordy, version.
http://amzn.to/xACNfl

50 Key Contemporary Thinkers
http://amzn.to/w3Wg19

Ways of Seeing
http://amzn.to/y88zBH

For an engineer it is quite different. Although there are obviously new and fresh topic which deserve to be followed on github.com for best practices on #PHP unit testing a milestone is http://www.amazon.com/Real-World-Developing-High-Quality-Frameworks-Applications/dp/0470872497/

I think books are good for sharing the "big" thoughts - those without expiration date. If you're fascinated by how our mind functions, I would recommend Daniel Kahneman "Thinking, Fast and Slow".