At the moment when you view a Challenge you have joined, the books appear to be sorted in the order they were added.

Add a sorting option.

• Added order
  • I assume this is the current default ordering, and would represent the order they were added to the Challenge
• Read order
  • eg grouped Reading, Read, Unread [per user]
• Unread order
  • eg grouped Unread, Reading, Read [ per user]
• Priority order
  • this would be if you allowed the user to drag and reorder the book entries, but obv would require more infrastructure than simply supporting the grouping options above

Rationale: If there are dozens of books in the Challenge and I've read some number and maybe have a couple others in the middle of Currently Reading, it's fiddly to either easily see the ones I've done, or to see which ones are still to be read.

It's possible this suggestion is mostly of use for a Challenge which is to Read a Number of books from a selection.