i am a cs graduate student interested in bioinformatics research. i don't have a good experience with biology , so what is the best bioinformatics book for a computer scientist?
You should look at the "books" tag over at the bioinformatics stackexchange, called Biostar. It has several threads that list good bioinformatics texts.
If you are interested also in Machine Learning I will suggest :Bioinformatics: The Machine Learning Approach.
After said that is important to ask you to which field of biology you'd like to approach as a computer scientist. Since genetic is very popular I can suggest for now Introduction to Computational Biology.
When you say 'I do not have a good experiece with biology' you mean you had some trouble with it in the past or you do not know a thing about it? (in the last case you'd better start to a common biology book for high schools)
I would suggest Bioinformatics, from Polanski and Kimmel (Springer, 2007). Very good introductory material on statistics, computer science as applied in bioinformatics. It also covers the necessary grounds in computational biology.
While a bioinformatics book is certainly great, I would argue that you would benefit from a proper biology text book. It's bound to be better in explaining the biology than any bioinformatics text book, and any biologists you will have to interact with in the future will be thankful if you have a clue what they're talking about.
In my view, a good general book on Molecular Cell Biology is by Lodish et al.
As far as bioinformatics goes, I quite like this one and this one.