CSS layout is so flexible, you could spend a lifetime exploring the possibilities. And some people seem to be doing just that. You can gain from their labors by reading articles, checking out online examples, and experimenting with tools that can do some of the CSS work for you.
Interactive CSS Box Model (www.redmelon.net/tstme/box_model). Fun, interactive tool for visualizing the box model.
On Having Layout (www.satzansatz.de/cssd/onhavinglayout.html). Not for the faint of heart, this highly technical analysis of Internet Explorer explains the main cause (and some solutions) for many of the CSS bugs that plague Windows Internet Explorer 6 and earlier (and some of the bugs that vex IE7).
Perfect Multi-Column CSS Liquid Layouts (http://matthewjamestaylor.com/blog/perfect-multi-column-liquid-layouts). Great examples of many different types of liquid (full-screen) layouts that work on everything from IE 5.5 to an iPhone.
In search of the one true layout (www.positioniseverything.net/articles/onetruelayout). Interesting—if slightly mind-bending—presentation on how to create a float-based layout that overcomes most of the limitations of floats.
CSS-Discuss Wiki page on float-based layouts (http://css-discuss.incutio.com/?page=FloatLayouts). Even more links to float-based layout resources.
CSS-Discuss Wiki on Absolute Layouts (http://css-discuss.incutio.com/?page=AbsoluteLayouts). Good resources with some helpful background information.
Learn CSS Positioning in Ten Steps (www.barelyfitz.com/screencast/html-training/css/positioning). Quick, hands-on overview of CSS positioning.
Making the Absolute, Relative (http://stopdesign.com/archive/2003/09/03/absolute.html). Guide to using absolute positioning for subtle design effects.
CSS Layout Generator (www.pagecolumn.com). Pick the number of columns, tweak a few knobs, and this website generates all the HTML and CSS required. With a site like this, who needs a book (just kidding).
Even More Layout Generators (www.webdesignbooth.com/15-extremely-useful-css-grid-layout-generator-for-web-designers). If you can't get enough of websites that automatically create your CSS and HTML, you'll find a list of 15 different online tools.
960 Grid System (http://960.gs). One of the better CSS frameworks that provides a set of basic styles and a technique for using divs and class names to create complex, multicolumn, fixed-width layouts. (You can find a detailed video introduction to this system at http://nettuts.com/videos/screencasts/a-detailed-look-at-the-960-css-framework/)
YUI Grids CSS (https://developer.yahoo.com/yui/grids). Yahoo's very own CSS-layout system. It's a bit techy, but like the 960 Grid System above, provides a basic framework for building complicated multicolumn layouts.
Blueprint (www.blueprintcss.org). Another popular CSS framework.
Intensivstation Templates (http://intensivstation.ch/en/templates). Cool templates, weird domain name.
Adaptive CSS Layouts (www.smashingmagazine.com/2009/06/09/smart-fixes-for-fluid-layouts). Provides many resources for building flexible layouts that adapt to the full width of the browser window.
One clean HTML markup, many layouts (http://tjkdesign.com/articles/one_html_markup_many_css_layouts.asp). Great blog post that takes a single HTML page and demonstrates eight different ways to lay it out with just CSS.
Variable fixed width layout (www.clagnut.com/blog/1663). Short blog post about a technique for adjusting the number of columns on a page, based on the width of the browser window.
3-Column Layout Index (http://css-discuss.incutio.com/?page=ThreeColumnLayouts). A nearly exhaustive (or at least exhausting) list of different three-column layouts.