Search engine optimisation is a mercurial industry at best and a spending black hole at worst. If you are planning to engage in SEO, either on a commercial level or to promote your own web site, it is vital that you stay abreast of the latest developments or your hours of work may be in vain.
There are a number of useful resources available online that can help you achieve anything from a basic knowledge of SEO techniques to a professional level of expertise. Studying and learning from these resources is an excellent place to start if you are planning to deploy the power of SEO - although they can provide no substitute for rigorous testing. As mentioned above, search engine optimisation moves extremely quickly and some of the information you find online may be outdated.
Perhaps the most widely recognised and used resource on search engine optimisation is the web site SEOmoz. As well as providing a number of tools and services that can help you plan and implement an SEO strategy, this web site features a handy Beginner's Guide to SEO that is free for everyone. There are few better ways to gain an understanding of search engine optimisation than reading this guide and a subscription to the site can provide more advanced users with plenty of food for thought. As an added bonus, the SEOmoz site also features a (free) daily blog offering plenty of useful hints and tips to the aspiring expert.
Another useful guide for the beginner can be found at BusinessInsider and is entitled '10 Basic SEO Tips to Get You Started'. This provides an excellent jumping-off point for beginners who go on to read the more in-depth exposition and analysis available at SEOmoz.
For more advanced SEO enthusiasts who have already gained a firm grasp of the basics, there are some interesting posts at seo-theory.com. Although the posts here tend to be more technical than those aimed at beginners, they are clearly written and suitable for those looking for an intermediate step in their journey towards expertise.
Most of the truly advanced knowledge can be gleaned from forums, paid courses or books, but it is extremely important to remember that since SEO moves forward so quickly, books on the subject are often outdated very shortly after publication. This is one of the major advantages of using the internet as your primary resource - posts can be written and published within hours (or even minutes) of a change to the search engine algorithms that determine relevance, value and thus rankings for web pages.
The web site stackexchange hosts an excellent Q&A based forum where users can ask specific questions and receive answers from established experts in the SEO community. The web site WarriorForum can hardly be overlooked by anyone who is serious about making money from internet marketing and search engine optimisation. Discussion is healthy and covers a wide range of subjects, including but not limited to SEO. Finally, a paid subscription to SEOmoz is a worthwhile investment for anyone with serious, well-considered plans to go professional.