Books:
o Sam-Ang Sam: “Silent temples, songful hearts: Traditional music of Cambodia"
More Information + Order: Here  
o Amy Catlin, Eran Fraenkel: “Selected Reports in Ethnomusicology: Text, Context, and Performance in Cambodia, Laos, and Vietnam
More Information + Order: Here
o Garland Encyclopedia of World Music: Cambodia
More Information + Order: Here
o Toni Samantha Phim: Dance in Cambodia
More Information + Order: Here

CDs:
The number of audio CDs dealing with “traditional” music is huge, this is why I only like to recommend CDs that I know.

o Celestial Harmonies:  “The Music of Cambodia“ (Box Set) 
More Information + Order: Here
o Chum Ngek:  “Homrong“ (overtoure music)
More Information + Order: Here
o Music of Cambodia:  “Solo instrumental music“  
More Information + Order: Here
o Ethnic Minority Music of  Northeast Cambodia
More Information + Order: Here

Please also check the Musicians page, most of them publish books and CDs too!