Of course, it's very easy to find journals which publish original mathematical work, and there are a number of book series that publish expository monographs. But if you have a work of original research which has reached monograph length, I don't know of many options. The only ones I know which are reasonably prestigious are Memoirs of the AMS and Asterisque. Are there other good options for publishing such work I'm not thinking of?
The European Math Society publishes EMS Monographs in Mathematics. There's also the Springer Lecture Notes in Mathematics, not to mention The Annals of Math Studies. These all fit in the rubric of `reasonably prestigious'. I think International Press publishes research monographs (at least they used to!)