what is the best online marketing 101 ebook for a tech founder having to market a product?
It's not an ebook but... my favorite so far (especially with startups in mind) is The New Rules of Marketing and PR. Don't shoot yourself in the foot by limiting yourself to ebooks!
I firmly believe that these days it is much easier for techies to get into marketing purely because a lot of the marketing techniques are getting more techy and online. Social media, blogging, Adwords, websites... good luck.