Here are some of the books for Web Desgining you can prefer.

1. The Web Designer’s Idea Book, Vol 1 & 2
2. HTML & CSS: Design and Build Web Sites
3. HTML5 & CSS3 for Web Designer Bundles
4. Learning Web Design: A Beginner’s Guide
5. Designing with Web Standards

Good Luck!

As you are already involved in btech studies, so it is not suggested to do web designing course along as it demands its own attention and is time consuming.

But as per your interest, if you wish to self study web designing then some of the books recommended are:

• The Web Designer's Idea Book by Patrick McNeil
• HTML and CSS: Design and Build Websites, by Jon Duckett
• Learning Web Design: A Beginner’s Guide
• Designing with Web Standards

You can also refer to online videos like:

• tutorialspoint.com
• codeacademy.com

Hope this is helful to you.

Good luck!