Here are some books on independent consulting skills I have been recommending recently.
For marketing
To get your marketing in order The Referral Engine: Teaching your Business to Market Itself.
And I wrote two books which are practical things you can do to market your business: Marketing your business without cold calling, and Eight more ways to promote your consulting business.
On market testing
To test a new concept or product, the chapters on market testing in The Four Hour Work Week.
