The book window was a device created by Catherine to fix Gehn's flawed descriptive and linking books. Placing one on the blank linking panel of an unpowered book would make it visible and functional. It is small enough to be portable and functions entirely on its own, unlike the firemarble domes built by Gehn for the same purpose, which require the golden superdome as a power source.
Book windows were made of a substance (apparently some kind of geode) that Catherine wrote into the Age of Tay. After linking to Tay for the first time, using one of the firemarble domes to power the descriptive book, Catherine manufactured some book windows from the substance and used one of them to link back to Riven. With the remaining book windows that she brought back with her, the Moiety were now able to use books without having to secretly access Gehn's firemarble domes.
All of the Moiety's books seen by the Stranger were made usable with book windows. The Tay linking book in the Moiety's secret tunnel on Jungle Island and the return linking book held by the effigy of Gehn already had book windows placed on their linking panels. When Nelah brought the Stranger a linking book back to Riven, she also placed a book window on the linking panel to allow the Stranger to use it.
One of the book windows was acquired by Gehn from a captured rebel scout; however, because of his low view of the Rivenese, he assumed that Catherine had fabricated the device by following an "explicit D'ni schematic" that she happened to bring to Riven and failed to realize that the material was from another Age.