I am going to do brief reviews of these two books since I devoured them both over the weekend, and am readying myself for book 4.
If I had to chose between them, A Monstrous Regiment of Women edges out A Letter of Mary by a hair, mostly because of Veronica Beaconsfield in the former. I enjoyed both of them enormously.
They are both still rereads. I know that I got at least past book 4 of the series during a previous read, but I can't - of course - quite remember where my series boundary is located. It doesn't much matter, in any event, because I read them both long enough ago that I can't recall the solutions, only remembering the barest outlines and plot points.
And, as is always the case, it is not the mystery that is the thing for me, with these books. It is Russell, herself, and Holmes, and feminism and theology. The historical research is impeccable, the setting so convincingly drawn that I am never thrown out of my full immersion in the text by an anachronism, that niggling interruption of "I don't think that could have really happened."
I am not a particularly devout person, but I am fascinated by theology. I found the aspects of both books that concentrated on Russell's theological research to be absorbing. But it is the suffragettes, and feminism, that have captured my heart. A Monstrous Regiment of Women dealt directly with feminism, while A Letter of Mary approached it indirectly, hinting at the utter devolution into chaos that would ensue in conservative Christianity if there were proof that Christ had female apostles. The intersection of theology and women's rights is one that is deeply interesting to me, and one which Laurie King riffs on brilliantly in both of these books.
The only thing that keeps them from both being full five-star reads is that the mysteries are merely pedestrian, and the solutions, at times, hinge on a confluence of really unlikely coincidences. But, over all, I can't say enough about King's accomplishment here - this is a truly remarkable series.