Hi Mostofa Bhai,
The most you can learn regarding usability (especially for web) is from the book "Don't Make Me Think" by Steve Krug. You can follow it by his next book, "Rocket Surgery Made Easy" to get a hands-on doing the stuff yourself. These books don't have any PDF version, so you can either buy the hardcover from store or buy the kindle version.
Don't Make Me Think is a book that every developer, designer, manager, well anyone related to a project with an user interface should read.
Regards,