Because the search engines seems to evolve at warp speed, the only way to keep up is to read the best SEO blogs - the value is that each blog author usually has an insight into what is going on that others have overlooked. Taken together they provide a good insight into how search engines work at a given moment.

Here are my favourite SEO blogs:

SEO Theory is the blog of Michael Martinez, who is Director of Search Strategies at Visible Technologies, Inc. in Seattle. He's been doing this since 1996, so has pretty much seen the search engine industry develop from it's infancy.

He tends to take a top level view, looking at strategy from the search engine's point of view, analysing the objectives of Google, Yahoo and Bing and then looking at how this affects web publishers and search engine consultants. This makes for a pretty deep read and if you are really interested in how search engines work, his blog is a must.

He's also very much in favour of optimizing for all the main three search engines instead of just optimizing for Google.

In my opinion, his best post is The Infallible SEO Option

Tropical SEO is the blog of Andy Hagans who founded the social bookmarking site for finance and media, Tip'd.

He started the blog back in August 2006, when he was a mere consultant at an SEO firm hoping to do big things on his own. His site is a little difficult to navigate as it doesn't have an archive - you have to use the previous post/next post thing to move about. However you can solve this by going to Google and typing in site:http://tropicalseo.com/ and the engine should return a list of all his posts they have indexed.

My favourite post of his is dated from 2007, just before he went on to bigger things - Is your site defensible

Leo Dimilo is an Internet Marketer who has been doing this job for ten years. His blog only started in late 2008.

He tends to analyse lots of the web marketing products being sold to newbies about how to make money online (such as PotPieGirl's One Week Marketing program, and the Keyword Academy course), and many of his insights as to whether they work or not are very useful for those thinking of joining. He too doesn't have an archive on his blog (what is it about these internet marketers? Is it some sort of SEO technique?) so you have to use the related post list at the bottom of each blog post to get around. Or you could go to Google and type in site:http://leodimilo.com/internetmarketingblog/ to get a list of all the posts they have indexed (luckily these guys are good at what they do and have most of their posts indexed)

My favourite post of his is Niche Marketing, How to select a niche

SEO Book is one of the longest standing SEO sites on the net. It's run by a group that sells SEO consultancy and training services.

However, included on the site is a blog, which is full of interesting insights into the industry.

My favourite post is a recent one - 20-25% of Google queries have never been searched before. Actually that's not the real title of that post, - but my re-written title gives you a better idea of the content!