This makes it sounds like a wizard would have a really hard time to prepare a spell from another's spellbook. Is that the case?
That is correct.
As you quoted, each wizard's spellbook notation is unique, therefore you can't prepare spells from someone else's book.
You must first copy the spell into your own book (deciphering the notation in the process), after which you can prepare it as normal.
Rules As Written: It's unclear
Rules As Intended: You likely know which spell it is
5th edition has tried to stay away from "GOTCHA!" moments, so it feels against the spirit of the game that they would make you spend time and money, only to find out that you don't want to copy "Pete's Portable Outhouse" into your spellbook, or whatever.
There are various modules wherein players find a spellbook "with X,Y, and Z spells" and no mentions of "after the wizard deciphers part of the spellbook, they realize that these are the spells in the book."
Additionally, that you could potentially start trying to copy down a spell that you can't prepare (a level 2 spell, while you're still level 1) would potentially be possible if you didn't know what the spell was. Would that mean that you've wasted the 2 hours per level and 50 GP per level up to what you could potentially prepare? Would the DM just tell you "You try to decipher the spell and fail."? Neither of those seem to be keeping with game intent.
Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level which you can prepare and if you can spare the time to decipher and copy it. (emphasis mine)
Rules As Fun (and Rules As Common Sense, likely): You definitely know what the spells in the book are
It is neither fun, nor interesting, nor interactive in any way to make players decipher what the spell is before they can decide whether or not they want it.
Just imagine being a first-level wizard and your DM telling you "You find a dusty tome in the abandoned wizard's tower. You have to spend at least 50 gold pieces each to find out what the ten spells within can do."
You'd sell the book for what you could and walk away (or at least I would).
...
Another interaction occurs to me. If you want to pay to copy a spell out of a wizard's book, you wouldn't actually know what spell you were copying until you were done. While hilarious once, it would be a huge jerk move on both the side of the wizard and the DM allowing it.
Best Answer
Simply Reading it.
You can "read" it if it is on your spell list just fine as you can read scrolls of spells on your spell list even if you don't know the spell. If it isn't on your spell list, you should be able to "read" it with something arcana check.
That doesn't mean you can grasp the spell from the spell book enough to cast it. This will tell you what is written in the book. You'd learn what spells are in the book and their general effect.
Casting From It
However, you can't cast it without fully understanding it. Which requires studying it and copying it into your own notation into your spellbook which requires time and gold.
And then preparing it:
Magic Item Spell Books
Some magic item spellbooks might be the exception to that. The DMG lists a few.
These are just fancied up spell casting foci that grant spells, benefits or other effects (Book of Vile Darkness can cause insanity for instance).