You imbue your eyes with a powerful filter that allows you to peruse the pages of an evil tome without automatically triggering its baneful effects against nonevil readers (if any). The eye of isolation does not benefit you if you are evil, and it does not allow you to gain the benefits, if any, of the evil tome perused. It does, however, allow you to learn the tome's contents in broad strokes. If a specific piece of information could bring forth horrific mental images, the eye of isolation blocks complete understanding so that you grasp only a general summary of that topic.
Avoiding baneful effects in this manner still bequeaths 1d4 periods of nightmares to good- and neutral-aligned manifesters. (At the DM's discretion, you may even visit the Shallows.) For instance, when the good-aligned psion Khara manifests this power, then attempts to page through the Book of the Shallows† (using inert metal rods to turn the pages), she does not fall prey to the baneful effects normally visited upon curious good-aligned readers; however, neither does she fully understand or grasp the contents therein, and so she gains no benefit. The broad summaries of the various fetid topics give her nightmares for the next 1d4 nights.
Good manifesters first tempted by evil usually begin innocently by gaining access to this power in a power stone.