Wizarding Challenges are sets of Stickers separate/distinct from the Registry collected from battling in Fortresses. To accumulate Fragments towards Stickers from Challenges, one must complete battles/floors from Fortresses.
Make sure to read on Fortresses for Tips and important information on how they work.
Why do I keep getting Books from battling in Fortresses?
The answer is simply, you haven’t battled to a high enough floor of the Fortresses. Fragments you obtained for Challenges’ Stickers are NOT influenced by the Runestone you use, but rather depend on the floor on which you are battling. Runestone will simply affect the number of fragments you obtain.
In order to unlock different Challenges’ Fragments other than Books, one must battle on higher level floors. Books are confirmed to be the common drops from the Ruins 1 floor to the Tower 1 floor. While one may obtain fragments from other Challenges’ categories from those floors, it is extremely rare.
Reports show that high level players, lvl 126 in Challenges for instance, are only starting to find Fragments from the second Challenges’ category. Two images below show an example of the various floors available in Fortresses. They go in that order:
Ruins > Tower > Forest
We need the community’s help to confirm which Fragments are obtained from which floor. Therefore, when battling in Fortresses, please complete the following Survey!
Completion/levelling up of sets of Wizarding Challenges will reward you with Scroll Books. Challenges are one of the four types of Foundables. As indicated in each of the graphics below, Challenges must be returned to specific sections of the Challenges’ section of the Registry in order to be completed.
Furthermore, thanks to Discord member, @Half_Blood_Princess, we also know the experience rewards for Wizarding Challenges, see Experience for all the details.