Undoubtedly one of the great things about open source is that so many handy programs and resources are completely free. You're unlikely to find a better deal on a comprehensive Ubuntu guide than the popular one by Keir Thomas, which you can download for free in PDF format (the print edition is available for 9.99 from Amazon). Ubuntu Pocket Guide and Reference: A Concise Companion for Everyday Ubuntu Use would be a great addition to your portable library. This edition covers Ubuntu 8.04 LTS, 8.10, "and beyond," according to its promo.
And while we're on the subject of good resources, don't miss Jack Wallen's post in the DIY blog: "Get top-quality open source security tools in one distro." Jack covers what's available in the Network Security Toolkit — a distribution based on Fedora that includes hundreds of tools for monitoring, analyzing, and securing your network.