I need to study more about common methods used in Biotechnology and proteomics like ELISA, chromatography, blotting. Does anyone know a useful book on these topics?
For most of biochemistry lab related techniques "Molecular Cloning: A laboratory manual" is generally considered as the bible. Also in this webpage http://www.molecularcloning.com/ you can find some sample protocols
You could also try the web. I like openwetware. It is a research Wiki started by researchers at MIT. You can find it at this URL: http://openwetware.org/wiki/Protocols.
The book that Tarkeshwar just mentioned Molecular Cloning by Sambrook and Russel is considered by many to be the molecular biologists bible. Also journals such as Biotechniques is a good place to find methods.
"Protein Purification: Principles and Practice" by Robert K. Scopes (and other Springer Advanced Texts) are great for this sort of thing. They teach the theory behind the techniques so that they really stick. The protein purification book I mention is only about $30 used these days and well worth it.
If you're getting into protein crystallography at all, "Crystallography Made Crystal Clear" is probably the best book I've read in that area. You may have to read more in depth, but nothing throws you into the field with more practical knowledge so gently.
For most molecular biology, I'd recommend reading the Cold Spring Harbor Molecular Cloning Manual. Most institutions should have some kind of access to it through their library directly or through Inter Library Loan programs. It's 2000+ pages though (and about 16lbs!) so be ready to do some reading. It's full of background information and detailed descriptions of how things work prior to actually laying out standard protocols for the techniques.
If you're looking into getting into synthetic biology "Synthetic Biology, A Primer" was a good book for teaching the principles behind thinking like a good engineer and a good biotechnologist and interfacing the two.
Bust most of all, read books and papers often and get experience in lab settings.