I have websites and have using SEO for it.

But my question is that how to improve SEO, is there any perfect and easy guide available over the interent for the SEO for website.

Waiting for expers answers.

No there's not. It takes a lot of hard work.

Sorry I'm afraid there's no silver bullet to SEO. It takes a lot of time and hardwork but be rest assured that in time, all your effort is bound to pay off. There are loads of resources online like the one you've already been given. But SEO is not a static thing and the rules are constantly changing so you have to update your knowledge regularly. Hope that helps.

You have to learn more no short cut method.
Try to learn step by step firstly learn Onpage optimization then Offpage Optimization.

SEO can be divided in to Onpage & Offpage.

Onpage optimization mainly discuss your content ...
How you organize your post?
What is your targeted key word?
keyword density in your article.
Use synonyms antonyms of keywords in your article.
Meta tag creation.

Offpage Optimization has a lot of word to say , simply i can say create more & more backlink.

Yes i will also recommend this e book.. I studied it 6 months back for my website and it does have really nice points which can help you in the long way!

An often stated approach would be to concentrate on that which is within your own power; your content in your web pages and how you optimize all that to encourage the search engine to believe your stuff is credible, unique and intelligent.

SEO is really a good process and not an easy task. You must read some seo tutorials first. SEO book may be a good solution for this.

Optimize your site on page and try to get backlinks from different sources