I would recommend "Start Small, Stay Small." It was very useful for me when I starter a side business that two years later has billings in the range of $175,000/year in licenses. In my case, it is subscription-only offline software.
If you are aiming for more than a lifestyle business, read The Curse of the Mogul [1][2]. It's a graduate level course in formal business strategy disguised as a fun and easily readable analysis of why the media industry is so broken. The author is a Columbia business school professor who specializes in business strategy. The book won't help you get your first customers but it will help you design a business that can stay on top once it gains that traction.
What specific areas are you looking to cover? Ryan Holiday is really great for covering marketing and I really like Alexander Osterwalder for building a suitable business model - the business model canvas really gets you to think about all the processes within your organization.
- The Growth Hackers book
- The books from Steve Blank
- The Art of Profitability
- The Checklist Manifesto
- iCon - The Second Greatest Act in Business
- The Narrow Road
- A good book on Copywriting
- The Mythical Man-Month
- Peopleware
- No Silver Bullet
- Death March by Edward Yourdon
- A Random Walk down Wall Street
We send out a book each month that matches the course you sign up for. The goal is to help people read more, but I also believe reading is one of the best ways to develop the soft skills needed in business. One book a month, every month, is compounding interest for your brain.
http://www.sideprojectbook.com/ This is an ebook I put together where I interviewed 39 entrepreneurs who had built a successful side business without leaving their full time gigs.