Sadly, I can only give it B+ for effort and good intentions. I was going to write a long review but I find that someone else has already said much of what I wanted to say and a good deal more, here.
I'd only add in Clothier's favour that when she sticks to politics and statesmanship, she's good; I wish she had jettisoned most of the romance and stuck to the nuts and bolts of being a 12th-century queen. (And one cheer at least for making her not only quite unappalled by blindings, castrations and royal hunts consisting of mass slaughter of trapped animals, but having (almost) no truck with 'How can I marry a man I do not lerve???' either.) She also sternly set her face against 'exotic colour', perhaps even too much - you don't get much feel for how Georgia and its people looked. The real failure in it is the Young Lerve element, which is not only, to my taste, quite unnecessary but just not even well done; she fails to make her Tamar 'n' Sos pairing interesting or particularly convincing.
I have to declare an interest here; I've always had a huge admiration for Davit Soslani, whose achievement I think is unique in history; in a male-dominated warrior society, he spent his entire career as the (brave and mega-capable) subordinate of his wife, and still managed to go down in their country's history as a great warrior-hero and king. Anybody know of an achievement matching that? I don't. I'd love to meet that man, if only between the covers of a book, but I didn't meet him in The Girl King and am deeply, deeply disappointed.
Another gripe; the notion that readers will warm to juvenile historical characters if their names are given snappy short pet-forms - Sos for Soslani, Zak for Zakari, Con for Constantine. This reader doesn't; it grates. And frankly, if you require your readers (as Clothier, to her credit, doesn't shrink from doing) to take in their stride a character with the surname Mkhargrdzeli, do we really need his given name shortened from Zakari to Zak in order to feel close to him?
And this isn't Clothier's fault at all, but she has been dismally served by her publishers in the matter of cover art. While Century's cover is just lettering that looks like something for a Tudor novel by S J Parris or C J Sansom, the Arrow Books one is crassly inappropriate. Clothier very rightly stresses in the book that Tamar never went into battle herself, but here we have a full-on Girl Knight in full 15th-century Western European armour - give her a white banner and that'd be a picture of Joan of Arc. It's a strong contender for this board's 'Rubbish Medieval Book Cover' prize.
Still, Clothier does give a small part in the novel to everyone's favourite bad-but-fun Byzantine, Andronikos Komnenos; and he has a major role in her second book, The Empress, about Agnes of France who was sent to Constantinople in 1182 to marry Alexios II Komnenos. But, having just sent it to Silverwhistle for Christmas, I'll leave her to review that one!