Unfortunately, we are not going to dive further into the CSS preprocessor as it is actually beyond the scope of this book, and there are many other CSS preprocessor features that are yet to be covered. So if you are interested in the subject, I recommend you refer to the following sections.
To learn more about LESS, refer to the documentations mentioned as follows:
There is no better place to start learning about LESS than its official documentation (http://lesscss.org/#docs). It covers anything you'll need to know about LESS right from the basics. There are also some examples provided to implement the languages.
Over at Webdesigntuts+, Daniel Pataki (http://danielpataki.com) has covered LESS in depth and even provided more useful examples (webdesign.tutsplus.com/tutorials/htmlcss-tutorials/get-into-less-the-programmable-stylesheet-language/).
Oliver Caldwell has shared some tips in his post (http://oli.me.uk/2012/02/25/getting-started-with-less.html) on getting started with LESS. He has shown how to run the LESS compiler with Node.js (http://nodejs.org/) and NPM (https://npmjs.org/).
To learn more about Saas, refer to the documentations mentioned as follows:
The Sass documentation is immensely comprehensive, but for me it often works as a good sleep inducer. So, I would suggest The Sass Way (thesassway.com) for you to start with Sass from the beginning.
If you prefer video rather than a text-based tutorial, you can follow the Youtube playlist (www.youtube.com/playlist?list=PL2CB1F80266E986EA) from LevelUpTuts that covers Sass thoroughly.
Chris Coyier has shared a screencast on the introduction to Sass and Compass (http://css-tricks.com/video-screencasts/88-intro-to-compass-sass/) at CSS Tricks.
Note
For further reference, Jonathan Verrecchia (http://verekia.com) has shared a good presentation slide about CSS preprocessors (http://www.slideshare.net/verekia/deep-dive-into-css-preprocessors). A few points that he has discussed in this presentation include CSS's limitations, a comparison between LESS, Sass, and Stylus, and also which CSS preprocessors you should use.