There are 3 books in the I Married a Billionaire series. The newest book is The Prodigal Son which came out in 2013.

1
I Married a Billionaire

Book cover of I Married a Billionaire
Maddy Wainright has given up on marriage - and men. After dating way too many losers who aren't capable of appreciating her for who she is, she's about ready to resign herself to a life of spinsterhood. Until her insanely rich boss Daniel Thorne comes along with an offer she…
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2
Lost and Found

Book cover of Lost and Found
When Maddy Thorne agreed to marry her billionaire boss in order to keep him in the country, she never would have guessed they'd actually stay together. But over a year later, she's still happily married to the slightly eccentric, maddeningly attractive Daniel. As he begins to come out of his…