Gumroad has a feature called PDF stamping [1] that puts the buyer's email address on the PDF. It's not perfect security, anyone who wants to ignore your copy rights probably will, but it might deter unthoughtful sharing.

Also consider a team/company license priced at some multiple (15x?) that will allow a manager or lead to buy your book for their company/team without having to worry about violating copyrights or managing buying the book for every employee.

[1] - http://blog.gumroad.com/post/40614820182/introducing-pdf-sta...