Should you need to read the books in order to understand a game's lore/characters?
No. If a game is attempting to put forth a story, it should give us the information necessary to make sense of its lore. Plenty of games exist which give little care to presenting a story - Such as some fighting games, action games, etc. And for these, where the gameplay itself takes precedent over everything else, having no real presence of story is fine. Not great, but fine. Having things like books aren't a bad thing if there is a sensible story already there, and books exist to flesh out the universe for those people who really do want to learn more...
But that should be no excuse for leaving a story totally absent within a game. How bad can it get?... I'll highlight one specific game, which blunders the concept of introducing players to its lore in a spectacular fashion.
Destiny
It's not quite what you are regarding to in the OP, in the sense that the game doesn't really have "books" needed to purchase in order to come to grips with the lore. Rather, the game seems to intentionally omit a majority of its own lore and relegating it to "grimoire cards." Basically, as you play the game and encounter enemies/locations/bosses/weapons, you uncover info cards for them which give pieces of information that flesh things out.
Not the worst concept on its own. But the problem is that the grimoire cards are utterly inaccessible ingame. You need to go straight to Bungie's website, log in to your account, to look at your own grimoire cards. Or you can use the mobile app. Cause that's what gaming sorely needed! /s
You have this database of information
This Library of Lore
A treasure trove of history.
And you can't even
look at it ingame. Bizarre that a game whose lack of story has become a punchline has made no effort in all the DLC's to fix this.
The expansions have fixed this somewhat. House of Wolves introducing a villain with goals. Then the Taken King moving the concept even further by having a persistent antagonist with motivations and a strong presence. You know who he is, you know why he is there, you know how he operates - And this is all done within the game's own presentation through cutscenes and dialogue.
Destiny is now actually attempting the bare minimum of storytelling. It does, however, make the grimoire card problem even
worse.
Although there is now a storyline. There is also a slew of new Grimoire Cards which are more important to the lore than ever.
Altogether, the new grimoire cards are referred as 'The Book of Sorrows.'
And they entail a thorough history of not only the entire Hive Race (the major Antagonist race of the Taken King), but also major background history of various other races, the 'darkness', and the Traveler itself. The hive origins, the planet they came from, who Oryx is, how the Hive Religion functions, the progenitors of 'Darkness' and how it operates and the motivations of the Hive Worm Gods. The Traveler's questionable role in spreading darkness throughout space. The origins of the Vex Race (A total fucking mystery in the base game, and were also the major antagonist species before the expansions released)
All of this insanely beneficial information which explains not only the origins of damn near everything - But also explains much of the gameplay logic...
Totally inaccessible within the game itself.
So yeah... Please give us
something. And if there's more beyond cutscenes and cursory dialogue, please let us see it in the form of an in-game codex/database/library. Chopping story out of a game to a point where it makes no damn sense on its own is inexcusable.