After opening at least 20+ Hogwarts Treasure chests, I still haven't received a spellbook.
@SiriusAZ I'm guessing they are a 5% chance, like the Trace charm. The other thing to be certain of is, are you counting the correct trunks? There has been confusion from people before thinking that Hogwarts is the symbol with the "H", but that is actually Legends of Hogwarts. Hogwarts is the red 9 and 3/4 icon.
I just purchased it myself and received one book from 22 trunks. It dropped on the 10th.
@Attrition I'm still waiting for the first one. I'm guessing I've opened 30+ red Hogwarts chests so far. I'm not sure the chests are worth spending the DADA books on.
@SiriusAZ I got one on my tenth chest, so I can confirm that they do drop. I've opened another 15 without one, but 25 chests isn't enough of a sample to confirm anything.
I continue to suspect that it is 5%, like the Trace Charm, but I will need more data to support that. If so, although we will average one book every 20 chests, my data from the Trace Charm shows incidents ranging from 1 to over 100 chests between each.
I bought the Forbidden Knowledge one and it has given me NO spell books of any kind. Considering how long it takes to get the restricted books to get this, I'm not going to be buying any more of these. It's just a rip-off.