Cheers! I am happy to be of help. You could make a post on the Elder Scrolls Online forums and make a poll if you need to get data from a larger set of people, unless you've done that already. Also the Reddit threads etc of course. If I can help at all let me know. I am always happy to see research conducted on games/see games taken seriously as a medium and art form
Oh some other ideas occur to me; in-game the player characters create journals, and in-game books can start quests, which may or may not be 'marked' in the journals; some are Easter egg type quests. So that's two other ways books are incorporated directly into the game that the player experiences. And in ELder Scrolls Online you can unlock a codex which stores all the texts you read. The ingame explanation is that it's a spell that lets you remember with photographic memory, so the codex, like the journal, is a kind of diegetic record. ESO also lets you find books in the overworld and complete sets, after which you can purchase the set as furniture items for player housing, so you can make a library of your own design.
I know a Skyrim mod that added libraries in the game world that included vendors that sold all the in-game books, so clearly there is demand to have the game world books available to read. And of course, there is the Daedric realm of Apocrypha ruled by Hermaeus Mora, Prince of Knowledge, where Black Books (in true Lovecraftian fashion; Mora himself has a lot of Cthulhu in him) magically transport the player to Mora's realm, which is a vast library where he hoards all knowledge he can get his tentacles on. In the Dragonborn DLC for Skyrim you learn bits of knowledge along the quest, finding more books and exploring more areas of Apocrypha.
There are also examples of scholarship within the game world; historians debate each other, there are essays and commentaries elucidating other texts etc, and you can meet authors in the game too.
Writing and literacy themselves have some history and commentary in the game world of the Elder Scrolls. There is a god of logic and reason, Elder Scrolls (which are holy artefacts after which the series is named) which are sort of quantum records of history that can be used to prophesy to an extent but which are also uncertain in what they record until the Hero makes a choice or effects an outcome, after which the quantum wave function of multiple possibilities collapses, so to speak, such that the Scrolls become a magical indelible record of the historical event in question. In terms of interacting with the Elder Scrolls, in Oblivion the final Thieves Guild quest has you steal one, and you use a few in Skyrim for different questlines, reading them etc.
There is Mora, a demonic entity who hoards knowledge, an Elven god scribe, Xarxes, and in the Song of Pelinal it is recorded that some humans opposed writing things down ('Nonscriptionists'), who a demigod on the side of humanity killed and wrote with their blood as an example.
Related to interacting with texts in game, there are alternate alphabets such as the Daedric script which people translate to reveal hidden messages put in by developers. And some books explicitly contain riddles and secret messages. For example, the first word in each paragraph of Commentaries on the Mysterium Xarxes forms a secret message, and the first illuminated letter of each paragraph contains yet another message; all of it to help you locate the cult. And the 36 Sermons are simultaneously a piece of theocratic propaganda, the man-god Vivec's self-aggrandising epic poem, a source of skill point increases, a magical retroactive alteration of Vivec's own timeline, a guide to the reincarnated spirit of someone he betrayed and which prophecy (referenced in the game's opening text) and the game's main questline reveal is the player character, a source of lore, and contains a coded message that is an admission of guilt; that Vivec and his co-conspirators did indeed murder the person who has since reincarnated as the player, though the player finds this out only later in the main questline, thereby learning about the prophecy and simultaneously making it true in the process of playing the game, by fulfilling the requirements of the prophecy. Then there's clever tricks like with the book The Dragon Break Re-examined; the god of time is a draconic figure, and a Dragon Break is a rupture in time, and has happened only a few times in the history of the game world. The book argues that these recorded 'dragon breaks' are nothing more than errors resulting in historians moving from one calendar system to another, etc, that is, non-supernatural means. What is hidden is that the book references events in the past tense that would actually date the book in the future, and the in-game author's name is an anagram of 'Darn Fool'.
The Black Books themselves contain snippets of poetry etc that are intertextual references, such as to Lovecraftian fiction or works in a similar genre, such as through referring to Robert W. Chambers' King in Yellow, or refer obliquely to the in-game concepts like 'Chim', itself a philosophical-religious concept in the deep lore of the Elder Scrolls universe that has links to existentialism, Nirvana and other real-world concepts, and is a state of mind/godhood that was quite possibly achieved by two non-player characters in the game universe. and is hinted to be them in a way realising they are characters within a game world, and thus gaining access to 'cheat codes' by which they manipulate both the timeline and material reality of the game world. So in other words you have in the game world books telling players about non-player characters who ascended (or in one case, wish to ascend) to a state of 'godhood' that is either using cheat codes within the game world, or wanting to become a player character themselves, in the sense that they could be all genders, all moralities etc at once, just as the player can choose to do be anyone or do anything in the game.
(Edits: grammar etc)