Some years ago I was in the same situation and I ended up writing my own system. That said, there are a few libs you can use. I have had all of the following integrated into my engine at one point or other:
https://github.com/garrynewman/GWEN - GWEN is a very simple UI layer with nothing extra. I don't think there is a DX11 renderer, but it should be farily simple to implement.
http://cegui.org.uk/ - CEGUI has been around for a long time, it is mature and diverse, but perhaps a little old-fashioned. Also it tends to bloat your project quite a bit.
http://www.awesomium.com/ - Awesomium is really an offscreen HTML5 renderer rather than a UI library, but it can very well be used to do UI. In fact, that is exactly what I did and I enjoyed it quite a bit. However, I cannot use it anymore since they don't provide any 64-bit windows binaries. Also it is not open source.