A standard reference is "On Construction and Identification of Graphs", ed. by: B.Weisfeiler, Springer Lect. Notes Math. Vol. 558 (1976) ISBN: 978-3-540-08051-0
There you will find details on Weisfeiler-Lehman algorithm (called stabilization there, IIRC). Although it's not an easy read.
A modern reference with some details on various "generalised" isomorphisms is here.