I really really appreciate it if you guys suggest a brief and practical book(s) that can get me off. I hope the suggested book(s) deals effectively with customization of the themes sold here in ThemeForest and the book(s) includes PHP, MySQL too.
In fact, I just wanna concentrate on theme customization (no theme authoring right now.)
I am hugely grateful for your consideration.
Just read the WP Codex.