It depends on what and on how you read. There are lots of books with very inaccurate information in them. And reading a book does not guarantee that you will remember what you read when you need it.
The word, "well informed" is a relative term, too. I'm very well-informed about writing and about literature, but I'm very poorly informed about cricket. So obviously, you want to read well-written books about topics that are important to you, and you want to do something with the information you read to make sure it stays in your head longer than the time it took to read the sentence out of the book.
Do this, and you will be well-informed.
Personal experience--College professor/MA English literature