Which is grammatically correct?

Hello

Would you take a look at the following sentences?
1) This is the best book I have ever read.
2) This is the most difficult problem I have ever had.

In these sentences "ever" is used.
A grammar book say that if the subject are "This is the first / second etc. time that ..." or "This is the only ..... that ...", " ever" is not used in that-clause. However, I found that " ever " is used in these sentences.

Which is grammatically correct, with or without "ever"? Or are both correct?

Thank you.

This is the first time I have done it.
This is the first time I have ever done it.

They are both OK- the use of 'ever'is to give emphasis. It's unnecessary, but not incorrect. ;-)