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An RSS feed is a file containing brief announcements and links to web pages. It can also contain an "enclosure" as a link to a file. This is how podcasting works, the enclosure is a link to an audio file (the podcast).
An RSS aggregator is a program that regularly goes and checks multiple RSS feeds to see if there are any new announcements, saving you the trouble of periodically checking many websites, and frequently not finding anything new.
Here's how our RSS feed works. Copy the link location for the little orange button to the left (right-click in most browsers). This then needs to be pasted into your RSS aggregator. This act is known as "subscribing" to the feed. Once subscribed, your aggregator will periodically check the RSS feed I maintain for the book. When I release a revision, the aggregator will show you a brief announcement, and you can visit the download page to pick up a new copy.
I will also include enclosures, which will be links to the PDF version of the book formatted for electronic viewing (rather than printing). If you use software to pick up podcasts, that same software should also then download the PDF of the book, meaning you always have a copy of the latest version. Thus, the term "PDFcast."
As an extreme example, you can use iTunes to regularly pick up copies of the book. Under the "Advanced" menu, choose the "Subscribe to Podcast" menu item. In the dialog that results, paste in the URL for the feed (its right there in the next paragraph). Set iTunes to check for new podcasts at regular intervals (Edit, Preferences, Podcast tab) and revisions will appear mixed in with your music. In the Windows version of iTunes, double-clicking in iTunes on the book will open it up in Acrobat Reader.
If you want to copy the correct link for the feed directly, here it is: