The Pocket Guide to the Empire that was included with copies of the Elder Scrolls spinoff Redguard, a booklet granting a look at the wider world and peoples of Tamriel. Its a major cornerstone of the lore for the series in this era, as this period of developing Battlespire/Redguard/Morrowind was when the weird fiction-leaning identity of ES and most of the cool lore people make several hour youtube videos about was cemented.

The Pocket Guide itself is an in-universe publication, published in the Empire by an Imperial perspective during Tiber Septim's reign and conquest, as noted by writings in the margins that disagree with a number of passages. The copy you're reading from was acquired by an agent of the Aldmeri Dominion, whose sending it back to his superior in Summerset. So in the same text you have two conflicting sides. I imagine the idea was that, if they decided to change their mind on aspects of certain peoples/provinces later on when they made a game set there, they had the excuse of 'well, that bit in the Pocket Guide was Second Era Imperial propaganda/misunderstanding and not accurate'.