"On Wings of Devotion" by Roseanna M White - The Codebreakers, book 2

All of England thinks Phillip Camden a monster--a man who deliberately caused the deaths of his squadron. But as nurse Arabelle Denler watches the so-dubbed "Black Heart" every day, she sees something far different: a hurting man desperate for mercy. When their paths twist together and he declares himself her new protector, she realizes she has her own role to play in his healing. Phillip Camden would have preferred to die that day with his squadron rather than be recruited to the Admiralty's codebreaking division. When an old acquaintance shows up and seems set on using him in a plot that has the codebreakers of Room 40 in a frenzy, he may just get his wish--now when he has finally found something worth living for.

I really liked both main characters. Ara surprised me in that she is willing to be so real with her emotions (that was a glorious punch to the nose!), yet the moment she reins in her hurt and temper, she has such grace and forgiveness for those who don't deserve it, and she's forever stepping out in faith with. And through every grouchy, bad-tempered action, Camden is just begging someone to believe in him, even when he doesn't believe in himself. A classic bad boy in need of saving, though he thinks there's no hope of it. And my favorite supporting character goes to Ara's father--he may have his faults, but he's a delightful eccentric that would fit in with any number of Disney single dads.

Then, of course, there's the thrilling setting--WWI London, when Germans were still sneaking across the Channel to bomb the city; spies abounding, cryptographers madly working in secret to spur on the allies and foil the Central Powers. I enjoyed the Mata Hari-esque spy, though with her own special talents and weakness.

Cameos from previous books abound, both from the Ladies of the Manor series and Shadows Over England series; nothing that would require one to have read them first, though--it stands well on its own (though they are all worth reading.).  Highly recommended!

The Codebreakers (third chronologically)
1. The Number of Love
2. On Wings of Devotion
3. A Portrait of Loyalty

Relates series:

Ladies of the Manor (first chronologically)
1. The Lost Heiress
2. The Reluctant Duchess
3. A Lady Unrivaled

Shadows Over England (second chronologically)
1. A Name Unknown
2. A Song Unheard
3. An Hour Unspent

Codebreakers comes third chronologically