We introduce a family of Jastrow pair product states for quasi-one-dimensional spin systems. Depending on a parameter they interpolate between the resonating valence-bond ground state of the Haldane-Shastry model describing a spin liquid and the (dimerized) valence-bond solid ground states of the Majumdar-Ghosh spin chain. These states are found to form an excellent basis for variational studies of Heisenberg chains with next-nearest-neighbor interactions and bond alternation as realized in the spin-Peierls system CuGeO3.