Can anyone tell me about a good book to unserstand php related to wordpress theme design

honestly, building a wordpress theme requires very minimal php knowledge, all of the functions has been built you just need to wrap it with your template

I'd agree. It's good to be able to read PHP and have some understanding of it. Any PHP book will give you that. I find the best information about creating themes is in the WordPress Codex itself and also on WordPress related blogs.

A few searches for things like 'WordPress theme tutorial' will bring back a lot of results. Search the codex for The Loop. That's probably the most important thing to understand. Also search the codex for Template Tags. You'll find those useful as well.

Just knowing the loop and template tags is really enough. After that it's understanding what each file in a theme does and which are necessary.

I'd start with the Codex. It really does have good info when it comes to building themes.