“Choosing A Ham Radio” is found as a supplement in the back of the Ham Radio License Manual, or you can download it as a PDF document.

If you are looking for more information about modulation, start with the Wikipedia entry on Modulation.

The squelch function is discussed more completely in the supplement “Choosing A Ham Radio” listed at the start of this section.

Instead of a separate device, many digital modes use the sound card built in to all recent computers to connect them to a radio.  The article introducing the digital mode PSK31 illustrates the typical use of a sound card for this purpose.

The four-part series of articles about digital modes will provide lots of background information about these popular forms of digital communication used by hams.  You can find more information about using digital modes at:

Winlink - organization that has developed and maintains the worldwide ham radio email network.
APRS - the home page of the Automatic Position Reporting System
TAPR - sponsor and organizer of digital communications initiatives

And in this print reference, ARRL's HF Digital Handbook .

You’ll want to be sure you understand the FCC’s Part 97 rules (Part 97.113 – Prohibited Transmissions) on commercial content and use of Amateur Radio.

If you’re interested in operating from a vehicle, be sure to bookmark KØBG’s Web page on Mobile Amateur Radio.  Alan is an enthusiastic and prolific author on the subject.

Given the interest in emergency and disaster-preparedness, there are a lot of on-line resources on this and related topics.  Start with the How Stuff Works pages on generators and inverters.

A comprehensive Web site on the subject, Battery University is a resource for many questions about batteries.  In addition, the ARRL Technical Information Service page on batteries has several ham-specific articles and links to more battery resources.

Managing interference is an important part of successful ham radio – bookmark the ARRL Technical Information Service page on RFI and EMI for its many useful articles and tutorials. A fine, if somewhat technical, tutorial by K9YC on the use of ferrite components for interference suppression is available online, as well.

This is the link to the referenced page on RF Grounding.