It might be just a guess, but could it be that the longer the Booktastic bookshop has no books, the rarer the book is going to be that is appearing next? Or is it just completely random?
I waited now for about 17 min and saw the Grundo Beauty book appear.
It is now the 3rd time that I saw it. I always tried to get it but was never able to. How do other people get them so much faster??? I refreshed almost every second befor that and clicked right away on the book and it was already gone, is there a trick???
I am currently collecting booktastic books too. I have by now 61 and maybe get another one today 
On my pets profile page you can see which one's I still need. I hope I will get help in this Guild and forum to get more of these unbuyable books, so that I will be one day also up in the list.
.. and i have restocked some of the books on your list in the past...