The Last to Know by Claire Highton-Stevenson

Being married to your best friend isn’t as easy as it sounds.

Grace Hart wants a baby. She’s hitting her late 30s with no children, no husband, and no prospects for the happily ever after of her dreams. But she’s decided the perfect partner to help her change all that and build a future is right in front of her in the form of her best friend of twenty years. And she won’t let a minor detail like being very straight and her bestie being a lesbian stand in the way
Caz Madden has no right to be attracted to her best friend. Yes, Grace is beautiful, and the two of them share a level of emotional intimacy and trust that only deepens as they begin a cosy domestic partnership, then marriage, that even their closest loved ones are convinced is a true romance, but that doesn’t make it real. Does it?
Against all odds, an irrefutable spark kindles between Caz and Grace as they share space and their lives, but giving into it could end up shattering their friendship and both of their hearts.
Blurred boundaries and growing feelings sweep them to the brink of a love more passionate and tender than they could have ever imagined. Will they let themselves fall, or will fear drive them apart?