Dear Mr./Ms. Executive Director,
I am writing to inform you that I am resigning as a member of (company)'s Board of Directors, effective (date).
As a shareholder with an investment in (percentage) of the company stock, I would like to (transfer the stock to another board member, sell back my stock to the company, retain a certain number of shares.)
Please contact me within the week so that we can discuss my stock options. Thank you.