Why did I get a free Kindle book?

AggieCouple

I was looking for some books for ML, and the Kindle version of one was free.  So I downloaded it (for my phone), and after that, there were no more free ones.  So, I got one free.  Why?  I have Prime, but only as an Amazon Mom member, which I thought didn't qualify for any of the supercool Kindle-related benefits.

ETA: I also do not own a kindle, just the phone app.