I can only repeat myself, sorry.
Yes, it's a rare book. It's random. (It might not have been the best idea to put it into an achievement but that's how it is now.)
Books (and other items) have a "level". So not all items can be found in all shops. This book is low level so you'll be able to find it in either Matras or Vastares (not in Mithria). Keep trying, you'll get lucky.