While this is a prequel, all of the books in the series (including this one) have a lot of "flashbacks" that reveal things that happened in Çeda's childhood, so any given book will have events that occur prior to the main timeline of the other books. Still, having read the first two books of the trilogy (and anxiously awaiting the next one), I would recommend against starting with this prequel, since there is character development in the main trilogy that would be missing, here. Still, if Of Sand and Malice Made was read on its own, without reading any of the trilogy, while there would be some things that aren't fully explained, I feel like there would be enough there to enjoy the story - and, perhaps, get one hooked on the trilogy - but it's not the optimal order in which to read them.
The book is comprised of three parts, each part continuing the story, but at the same time, very much its own well-defined portion, with its own events and conclusion. All three revolve around Çeda's unexpected and undesired interaction with an ehrekh named Rumayesh, who has taken an interest in "collecting" Çeda. As it turns out, Çeda doesn't want to become a host to an ancient desert demon, so she fights to find a way to rid herself of Rumayesh and her undesired affections, which will take resolve, physical endurance, inner strength, and a bit of research, to boot.
I highly recommend Of Sand and Malice Made to fans of the Shattered Sands series and, for that matter, hope that Bradley P. Beaulieu continues to add additional tales of Çeda's adventures outside of the trilogy, even after its completion. There's time in there for things to have happened, and Çeda is an interesting character. Again, I would advise reading at least Twelve Kings in Sharakhai before reading Of Sand and Malice Made.