I win every time on Easy, occasionally on Medium and never on Hard. I guess it means that the difficulty selector is calibrated correctly.
I win pretty much every time on hard. I have totally lost due to luck, though, even though I usually die to accidentally pressing the wrong key or just not noticing something obvious most of the time. I am sure it is possible to get to the book much more than 90% of the time. If you get the glitch where the book is unreadable then, yeah, you are screwed. But that has only happened to me once, I think.
Patience, mostly. The basic strat for hard is as follows: Get 10 progress so books start spawning. This isn't actually necessary for the run, but it saves boatloads of time and makes mistakes less costly. Avoid gaining any more progress. Find a room that has a good room layout to use as a base. That means it has two exits very close together and no long hallways librarians can spawn in that have view of the exits. You can actually do it with certain hallway-like rooms, but it is harder and more complicated. Once you have found such a room, take one of those two exits. If it goes to a hallway or you can't see any books, leave and use the other exit from your base. If not, flip back and forth until it is safe, then go check if the books are worth reading, never going far enough out that you aren't guaranteed to be able to get back to your exit if you see the lantern light from a librarian coming. Get resolve up asap, but don't read sanity lowering books if you are below 80 sanity. Don't go above ~35 progress until you've maxed your stats-- 100 sanity and 40 resolve. Then you start grabbing all the progress books you see, which slows things down, until you have enough progress that the book spawns (sometimes I give in to laziness and make a break for progress via catalogues here, which is risky, but usually (i.e. almost always) works out fine because 100 sanity is a lot of L space movement). You have to go for the catalogue in that room, which, unless you are incredibly lucky, means getting seen by Librarians and using L-space to evade them. Librarians come over to where they saw you last for a bit after they've seen you but before you enter L-space if you break LoS. Going one direction and then cutting across bookshelves at the last minute into another section of the Library usually works to clump most of the Librarians together, which gives you the space you need to read the catalogue. Unlike normal books, there's no penalty to reading your target book in L-space (you're stealing it for a client, not reading it yourself), so do that if it's convenient or just read it normally, whatever. Then get out of that room, either via your original entrance if you can in which case you win, or (this is much more common for me) out another entrance because getting back to the first one would cost way too much sanity and you are down to like 30-40. Use more sanity in the next room in that case to get to another exit, and from there the rapidly lowering difficulty should resolve the problem for you pretty quickly. Remember to go back and forth across screen transitions to refresh Librarian placement in rooms, and that if you find a screen with two exits close to each other, you can just walk back and forth for a while to end the game (well, you have to do one more room with a single Librarian that you can reroll as much as you like, but that hardly counts).
TL;DR: Get a room with two safe exits close together and use it as a base. Also make sure your stats are maxed, and that you never walk into an intersection where you might have ghosts on two axes.