R/C Model Aircraft Design, Andy Lennon, ISBN 0-911295-40-2 has a lot of info on slats and flaps on STOL wings.
I'm building an STOL wing for a BTE Flyin' King with fixed LE slots and slotted flaps and this book was useful.
Check into Eric Kler's site on Fubar hill:
http://www.fubarhill.com/ . He has an STOL section in "Tidbits".
MAN (Model Aircraft News) also has a book with some ideas for movable slats; I disremember the name of the book.
--Bill