Ok, so if we take the optional move, we put our tokens on the square we choose and move from there, right?

Assuming the above is correct - what if our roll day is the 4th? Do we take this move first, then roll, or roll then take this move, or does it not matter either way?

It doesn't matter either way - either roll first, and then take the optional move, or take the optional move and then roll. You can decide where you want to roll from.

The only caveat is that if you are going to roll first, then you actually do have to have finished your prior book before your roll. Does that make sense?

Yes. The post says that you don't have to finish your current book to take the optional challenge - but if you're going to roll first, then the new book is the one that you don't have to finish to take the optional challenge. Basically, you can be reading two books at the same time, but not three!

Do we need to start reading the book for this optional roll on the 4th? Like, say, if there's a particular book I want to read for one of these squares, but I have to check it out from the library, but the library's closed on account of holiday, and I'd need to wait until the next day to check it out?

Could I opt-in to this extra move, pick a book, but not start it until I can get a hold of it in a couple days?

Assuming the above is correct - what if our roll day is the 4th? Do we take this move first, then roll, or roll then take this move, or does it not matter either way?

Could I opt-in to this extra move, pick a book, but not start it until I can get a hold of it in a couple days?