This was a really poor and unnecessary ending to the series. The biggest point of intrigue in this book was Leah, and there wasn't any sort of resolution at all with her storyline. Who is she, what was she tearing everything apart looking for, what stone was she talking about, how does she know about the two worlds, which world did she originally come from? None of that was answered, they just locked her on Earth and said "well, I guess she's not our problem anymore so who cares."
Honestly, the parts about Leah and the diphtheria weren't necessary to the overall Dusk Gate Chronicles story. They were only there to fill in the rest of the pages needed for a book that was truly just trying to be about wrapping up Zander's new life in Philotheum (and a little bit about Quinn becoming a more confident queen). Honestly, there could have been an epilogue at the end of the last book about Quinn becoming a good ruler and Linnea/Zander getting together, and that would have covered the only important plot points in Blades of Accession.
If you've read the rest of the series then you should read this one just for the completion aspect, but prepare to be disappointed.