Here's a special offer for all ten of the people who have Windows 10 and buy ebooks from Microsoft.
Buy any ebook in the Windows 10 between now and 30 June and Microsoft will give you a $5 credit.
According to the fine print, the offer is open to US customers only and can be used once:
Limit 1 per customer. Customer must purchase a book between June 21, 2017 12:01 am ET and June 30, 2017 at 11:59 PM PST from Windows Store. Not valid on prior orders or purchases, or free books; cannot be transferred or otherwise redeemed for cash. Microsoft reserves the right to modify or discontinue offers at any time. Once purchased, an e-mail will follow within ten days with a promotional gift code. You must redeem code by August 15, 2017 and spend within 90 days of redemption. Gift code is valid in the United States only at select Microsoft digital stores for eligible purchases.
The ebooks can only be read in the Edge web browser, and that is only available through Windows 10.
So any ebook you buy would have limited value, but if you find one cheap enough this credit might be worth it.
Windows 10 Store via Microsoft