Once a time I sawheard someone tellingsay that books isare a way to access a knowledge from a person that you don't know you and is far away from you byin time and geographicallyspace.

1. Speech is heard, not seen
2. knowledge is uncountable, so doesn't take "a"
3. books are plural, so take "are" instead of "is"
4. "that don't know you" isn't quite right, "that doesn't know you" is the grammatically correct version, but also it's more natural to focus on the reader (you, in this case) than the author here, so swapping them around. Since that puts the subject right in front of "don't", "don't" is correct in that situation.

After hearing this, I noticed that I had read a lot of books from dead authors or old books which wasere written when I havewasn't born yet.

"was" -> "were": Books are plural

"Haven't (been) born yet" is present tense, and the been is required. But since you want to talk about a point in the past (when the books were written) it's you should use the past tense here. "when I hadn't been born yet" is another alternative, but I think wasn't is better here.

Probably tThis fact is probably obvious for the majority of people, but I didn't have this perspective.

"Probably" like this at the start of the sentence is a bit too casual for writing (but people absolutely will use it like this in casual speech).

OI once a time I saw someone tellsaying that books isare a way to access a knowledge from a person that you don't know you andand who is far away from you, byoth in time and geographicallylocation.

Once a time - 'Once upon a time' is often used to begin a children's story or fairy tale
'person that you don't know' = stranger

After hearing this, I noticed that I had read a lot of books from dead authors or old books which wasere written when I haven'tbefore I was born yet.

Probably tThis fact is obvious for the majority of people, but I didn't haveprobably obvious to most people, but I hadn’t considered it from this perspective.

OI once a time I sawheard someone tellingsay that books isare a way to access a knowledge from a person thatyou don't know you andand who is far away from you by, in time and geographically.

After hearing this, I noticed that I had read a lot of books fromby dead authors or old books which wasere written when I haven'tbefore I was born yet.

Probably tThis fact is probably obvious for the majority of people, but I didn't have this perspective.

I didn't have this perspective isn't "wrong" but I would say something like... "but this perspective is new to me".