Suitable introductory book on Bioinformatics for a computer scientist?
Systems biology and bioinformatics are quite diverse subjects, but here are some options:


Biological Sequence Analysis by Durbin et al.: the classic bioinformatics text for people with a CS background. Does not deal with networks.
An Introduction to Systems Biology by Uri Alon: A physicist's view of systems biology, focusing on design principles and networks. Interesting concepts and some math (no algorithms). Doesn't really deal with methodology.
Physical Biology of the Cell by Phillips et al.: Nice introduction to biophysics and quantitative biology (no algorithms). Many parts are relevant for systems biology.
Any diverse Machine Learning text will be relevant and many have examples from biology/bioinformatics.
Probabilistic Graphical Models are also quite popular in bioinformatics (e.g. Hidden Markov Models, Expectation-Maximization, Bayesian Networks). Probabilistic Graphical Models by Koller et al. is a very comprehensive CS book on the subject.


Another suggestion is to look for primers/reviews in relevant journals. This is important because the field is new, so once a new technology is developed, it takes time until the appropriate computational methodology is developed enough to make it into a text book. Also, they can point you to relevant research papers. For example, PLoS Computational Biology has an Education Collection which features a large variety of such reviews.