Oh boy, that's a difficult question. I still have to read the book myself, but you're probably supposed to read it after The Last Wish. Though, as somebody else pointed out, the epilogue is not a prequel, therefore I'd say you can read it right after The Last Wish or read it after The Lady of the Lake, the last witcher novel.

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The Wikipedia site may be of help to you. The books are all mentioned in the correct order that you should read them in:

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The Last Wish

Sword of Destiny

Blood of Elves

Time of Contempt

Baptism of Fire

The Tower of the Swallow

The Lady of the Lake

Season of Storms