The problem at hand (which secretly ties back to the previous two books, I think, or would if I remembered all those plot details from two years ago) is that a secret order of desert priests has captured a nest of shoggoths. They have shoggoth leakage. Leakage is bad, c.f. "Sea of Glass", "Irim/Iram/Irem of the Pillars" (my favorite Arabian legend that I never heard of until Tim Powers came along, thank you, Tim). Everybody on several sides wants a necromancer to solve their problems. They want her very, very badly. The necromancer deals with this by playing to her strengths, which are not what you expect.
I have decided that I like this (I think) trilogy (the third book ends at a good stopping point, anyhow). However, it's in a strange mid-ground between standard protagonist structure and ensemble casting. I wonder whether the whole thing would have worked better if Isyllt had been a pure NPC, seen only from the outside, with her companions carrying the story. No? I know, it wasn't my decision, I didn't write 'em.