Rob Swigart has kindly informed me that his book is now available through the Authors Guild Backprint program. You can now buy a paperback copy of the book. It was republished in October 2001, with ISBN 0595197841.
Portal was written by science fiction author Rob Swigart. Shortly after the release of the game, the entire text was re-published in book form as "Portal: A Dataspace Retrieval".
It's ISBN number is 0595197841. The book has been out of print since 1999, however the book often shows up on Amazon Bibliofind.
If you don't have the patience to read the story of Portal within the game interface, Portal was also published as a novel by Rob Swigart. There was a U.S. hardcover edition from St. Martin's in 1988 (ISBN 0-312-01494-5) and a U.K. paperback edition from Grafton in 1989 (ISBN 0-586-20649-3).
The novel uses the game's format of presenting the story as short fragments "written" by Homer, supplemented by information from other databases. I haven't compared in great detail, but the text appears to be mostly unchanged from the game.
The box contained three low density 5 1/4 inch discs -- a large amount of data for its time.
Also included was a booklet of loading instructions, a five page Prologue, the Worldnet Emergency Operating Instructions, a registration card, an order card for the hint guide, a product catalogue and, lastly, a poster-size (and totally useless...) map of the World Administrative Regions in 2077.