"Patch 1.21 (30 May 2016)
Improvements

Adds book titled 'A Miraculous Guide to Gwent' which displays number of gwent cards missing from player's base-game collection and information on where to find them, thus facilitating completion of achievement titled Collect 'Em All. Tome in question can be obtained either from gwent playing scholar in Prologue or from merchant near St. Gregory's Bridge in Gildorf district of Novigrad."

I found it. It was at a shop near the La Valette Residence in Novigrad

If only that were true. I started over and won several games against the scholar. He never gave me that book.

i found the book in novigrad, its next to the barber shop