For VBS quantum antiferromagnets in one dimension see also :

Ian Affleck, Tom Kennedy, Elliott H. Lieb and Hal Tasaki, Valence bond ground states in isotropic quantum antiferromagnets. Comm. Math. Phys., Volume 115, Number 3 (1988)

and

Stefan Knabe, Energy gaps and elementary excitations for certain VBS-quantum antiferromagnets, Journal of Statistical Physics Volume 52, Numbers 3-4, 1988