I loved ITSOL! Just reread it last month actually. I remember just being on the edge of my seat for the last third of the book If you enjoyed that I would recommend The Powder mage Trilogy by the same author, he does a good job of mixing a mystery investigation POV and a more action oriented one, just like in ITSOL, and I find it very engaging. Also The First Law Series. I think you'll enjoy the tone and mix of Known and Unknown Magics in those series similar to ITSOL.

I can also recommend The Will of The Many and The Blacktongue Thief, which were two of my books of the year for their releases, if you're looking for fantasy with a different tone than your standard, run of the mill sword and magic series