Sacred Hearts by Sarah Dunant

Virago

Heavy breathing in a 16th-century convent?
Kinda. Young, beautiful, madly in love and protesting with every breath, 16-year-old Serafina is shoved into the nunnery by her angry family.
Angry why?
Blame it on the boogie. They’d made a good match for Serafina, but she fell for the wrong man (wrong for her family, that is), and enters with a secret stash of letters To Ser With Love.
Mortifying
Which is what the convent proceeds to do to her. The Council of Trent is tightening the screws on over-indulgent nuns who wear makeup, keep pets, put on theatrical holy shows and consume wine and biscuits.
I didn’t think nuns were like that
We didn’t think nuns were like a lot of things. This St Caterina’s convent in Ferrara is raging with strife. Humble Suora Umiliana wants miracles, fasting, prayer and mortification of the flesh. Abbess Chiara, smoothly political, wants to keep things as they are.
Ah, life
Then there’s an aged nun who’s basically been in the slammer for most of her life, banged up in her own cell because her stigmata and visions are too politically exciting for Ferrara.
Holy God
All done in his name. Then there’s our heroine, Suora Zuana, herbalist and doctor, and the nearest thing you’ll find in the time and place to a rational human being.
And Serafina's boyfriend?
Wouldn’t want me to reveal the whole thing, would you? The good guys win in the end, but you’ll have to guess who they are.
Who’s this Dunant dame?
Multifaceted writer who leaps with effortless ease from noir thrillers (she’s a Silver Dagger winner) to The Birth of Venus, about a Renaissance babe torn between a dashing painter and her wise, kindly husband.
Should I take a vow to buy it?
If you like a book full of continual change and transformation, as the rule of St Benedict would put it. A bit too long, but the story is juicy.