I'd recommend 'Computational Chemistry' by Guy H Grant &
W Graham Richards, Oxford Science Publications, no. 29
in the Oxford Chemistry Primers series. Only #4.99! (1995)
You might also want to check out:
Computer Modelling in Molecular Biology ed Julia M Goodfellow,
VCH (1995) #72.00
Computer Modelling of Biomolecular Processes ed Julia M Goodfellow
& David S Moss, Ellis Horwood #45.00 (1992)
Dynamics of Proteins & Nucleic Acids by J Andrew McCammon &
Stephen C Harvey, Cambridge (1987) (bit out of date now though).
But the 101 text is undoubtedly Grant & Richards,
Jon Clewley