The 4 Steps to the Epiphany - Steven Blank
http://www.amazon.com/Four-Steps-Epiphany-Steven-Blank/dp/0976470705 This is the how to guide for building a start up and one of the foundation's of the "lean startup" movement. The premise is simple, dont build out your product until you have verified that people will buy it first. Not so simple in practice of course but that is why you need to read the book.
For me the most influential book to read was the The Mythical Man-Month. Yes, the book is more than 30 years.
The most important for me was to realize that even though the tools and the hardware are changing almost every day, there are still a lot that stays the same. I also realized that there are experience out there and that it is stupid not to read some books in nearby fields (psychology, marketing etc).
Ayn Rand's Atlas Shrugged and Fountainhead. Whatever your opinion of her writings, and those who over the years have claimed her for their own purposes, she held rationality to be the hallmark of what was good and right in humanity and ignorance, fear, and manipulation the basest of what we are.
Its a great book. Not business related, but a great book.
As for business books, I tend not to read those, but rather to read book summaries. They contain most of the info of the books, but are much shorter, and so it allows me to go though a lot of books in a short time (more time to work on my business :) )
I can't really say any one book changed my life - that is a bit overly dramatic. But one compelling book that is worth reading if you are involved in software development is "Peopleware "
It confirmed my beliefs on how a development organization can and should be run and helped me convince others of the value of treating people as people, not as interchangeable "assets" or "human capital"
I enjoyed very much "Micro-ISV: From Vision to Reality" (by Bob Walsh) and was one of the inspirational sources of my current entrepeneur adventure in software. The book exposes different perspectives, steps, considerations and real cases about the creation of little software companies.
link text As a developer, always was one of my ideas to create my own (packaged) software and stop doing what other people/companies wanted. Plus, I worked in consulting companies for 12 years and was thinking on taking a good (long) time for myself. Initially having the idea of a "sabatic year", then I had a better one: why not try to make my own business by doing what I like? Then I take my savings and drop my job to work full-time in my project.
Timothy Ferris' 4-Hour Work Week Made me realise I didn't want to be an employee my whole life and inspired me to follow the path of becoming a Web Entrepreneur. Timothy doesn't focus on the nitty gritty of how to get it all done but he certainly provides the vision and inspires you to "design your lifestyle. "
The Fifth Discipline, by Peter Senge.
This helped shape how I see the business world, what it can be like.
It also introduced me to many topics, such as dialog vs. debate/discussion and systems thinking, which helped to continue reshaping my world-view, as well as understanding what a mental model is.
Selling the Invisible - Harry Beckwith
Today, most of us sell things that are intangible (i.e. services, experiences, expertise), but most businesses still market these offerings like they were tangible products. This book helped me realize what was really important (and different) when marketing and executing for my business. Highly recommended.
"Getting Real" (http://gettingreal.37signals.com )
Having been developing web applications for 15-years, I think it is the book that most honestly describes the reality of the process. It has alot of solid advice about the processes and people involved in a building a successful web-based startup. It's straight and to the point and can be read within a couple of hours.
I probably come at this slightly differently as I came to the ISV industry from a completely unrelated base - the food wholesale market. The main book that influenced my thinking in running my businesses was:
Allen Leighton - On Leadership. UK businessman and former CEO of Asda (Wal-Mart in the UK)
I just finished Here Comes Everybody by Clay Shirky. While I don't know that the book will change my life, it does an excellent job of capturing some of the trends in technology that will definitely change all of our lives.
If you're working in or around social media, crowdsourcing, digital media, etc. - this is a great book. Many useful insights and an entertaining read.