Easiest book for modelling in general:
J.M. Goodman 'Chemical Applications of Molecular Modelling' RSC 1998. 540.285GOO
Useful guidebook about which methods to choose for what; contains no theory, just results of tests and advice about what they mean:
W.J. Hehre 'Practical Strategies for Electronic Structure Calculations' Wavefunction 1995. 530.411
Oxford Primer, a bit mathematical in places, but some parts may help understanding:
G.H. Grant and W.G. Richards 'Computational
Chemistry' Oxford 1995. 542.85GRA