Do you use this expression "If I'm not mistaken"?
If I'm not mistaken, this is the better book I have ever read.

If I'm not mistaken, he is from England.

Question

Do you use this expression "If I'm not mistaken"?
If I'm not mistaken, this is the better book I have ever read. 

If I'm not mistaken, he is from England.

Answer

It is used in two ways.

It is used if a person is fairly sure of something, but there is a slight chance they are incorrect.
"If I'm not mistaken, she was married before this."

It is also used if a person is absolutely sure of something and is using it for emphasis.
"If I'm not mistaken,  the value of pi is 3.14159."

Personally, I use the phrase when I am more than 92.7% sure of something. ;-)

Answer

Probably all English speakers have used the expression at one time or another, if I'm not mistaken. It is a parenthetical.