The Meaning of Marriage by Tim Keller is $2.99 on Kindle. (It's now $12.99...hope you didn't miss the deal! But still, a good price for a great book.)
No idea how long this will last. But I own the physical copy & multiple other copies to give away (and use for marriage counseling). And I still bought this for Kindle. You should too. This may be the most important book written on marriage (or that will be written on marriage) for years to come.
Grab it, and tell others!