Portfolio Overlay Resources

I have recently transitioned from IB to AM at a large US LP, and was wondering if anyone had resources on portfolio overlays. I am unfamiliar with portable alpha and portfolio overlay and am trying to do some reading on the topic but am not finding a lot online. I am most interested in how institutional LPs use overlays to correct for illiquid / PE overallocation due to relative performance versus the strategic asset class allocation. Any information would be greatly appreciated. 

I am currently thinking about overlays in three buckets: currency, asset class (the one I do not understand the most), and interest rates.