I have two recommendations. If you can track it down, get David Toop's book "Rap Attack" (or the second edition "Rap Attack 2"). This is a *solid* history of hip-hop and covers everything from griots and Jimmy Castor to Afrika Bambaata and . It has also lists of the 200 best singles. The second edition came out in 1991 so it trails off with N.W.A and the like and misses most of the bad stuff that followed.
One area no one has mentioned are those old Celluloid records releases. Earlier this year, I found a 3xLP set called "Trilogy" in 1985. The first two discs are African funk (Fela, Mandingo) and art funk and rock (Material, Massacre) and the 3rd LP (called "Beat Freaks") has 9 really obscure hip-hop/electro tracks. D.ST., the scratch artist on Hancock's "Rockit," is on 5 tracks and they all of the cuts have that sort of feel. Celluloid has re-released some stuff on CD recently so you may wanna investigate.