Alan Lomax's Folk Songs of North America has a brief tutorial for guitar and banjo in the back. It gives a couple of fingerpicking patterns. It also matches patterns to specific songs. It was a great practice guide for me (I wore the spine out on the book). Using the t=thumb, i=index, m=middle, and r=ring labels and placing them so the thumb covers the three low strings and i on G, m on B, and r on E mentioned above:

4/4 t/i/mr/i is a frequently used pattern.
    t/imr/t/imr mimics a flat pick strum

3/4 t/i,mr/i,mr
    t/imr/imr mimics a flat pick strum

You can combine them, alternating throughout the song.

4/4 t/imr//t,i/mr,i   

Roger in Baltimore