1. As to DEFINITE IMPERFECTIVE (Ехали) vs INDEFINITE IMPERFECTIVE (Ездили).
"Ехали" strictly denotes a movement with a single definite direction (Point A to Point B). "Мы ехали в Париж" means we were on our way to Paris. It is a unidirectional verb.
Here is an example when you can use it:
Когда мы ехали в Париж, мой друг учил меня французскому языку. - When we were on our way to Paris, my friend was teaching French to me.

When you mean a round-trip movement (e.g. Moscow-Paris-Moscow), you can never use "ехали". It is not a unidirectional verb.

"Мы ездили в Париж" means "We went to Paris (and then came back)".

2. Perfective vs Imperfective.

It is much harder to explain, as I wrote. But just keep in mind: Imperfective does not necessarily tells you the action was left incomplete. It rather tells nothing about its completeness. This is an important point.

So, "Я читал эту книгу" can be ambiguous in Russian. It can assume:
"I have an experience of reading this book in a certain moment in the past", and
"I started reading it did not finish".

The context easily clears it up.

And the sentence "Мы ездили в Париж" can hardly mean "we went to Paris but failed to reach it" So, it's a very natural way to express, and it just means "we have been to Paris".

If you want to describe some specific period of your life, e.g. how you spent your vacation, you are welcome to use the perfective form:
"Мы съездили в Париж" - we performed (successfully) our trip to Paris.

The difference between "ездили" and "съездили" is very subtle here. I can feel it, but do not know a good way to express it in English, sorry.


And as to your example,
"да, я знаю, я уже прочитал(а) её"
and
"да, я знаю, я уже читал(а) её"
are both possible and mean nearly the same.

I think this is a point where non-native speakers who know Russian well can drop more light than we, natives