There aren’t many books devoted to high lift systems, but a number of conferences has treated this topic quite well. Here are some of them.

1. Hoerner SF. Fluid Dynamic Lift, Hoerner Fluid Dynamics, 1965

2. AGARD. High-Lift System Aerodynamics, AGARD Conference Proceedings CP-515, Banff, Oct. 1993
   Review: A conference proceedings reporting the state-of-the-art in CFD (structured and unstructured methods, viscous-inviscid interaction), experiments (wake flows) and design of high-lift systems. There are also case interesting studies (most Fokker airplanes, Grumman X-29, Boeing YC-14, ATR-42). Recommended reading

3. Smith AMO. Aerodynamics of High-Lift Aircraft System, in AGARD CP 102, 1972.
   Review: Theoretical work on high-lift systems. Among other things, it explains clearly how the leading edge slat works and what are the theoretical limits for unpowered lift. Recommended reading.

4. Gratzer KB. Analysis of Transport Applications for High-Lift Schemes. AGARD LS-43, 1971.