How can it be alphabetical and then by level? No two spells are going to have the same name, so if you have a top level sort by alphabetical, that's it. There is no second or third level sort. Unless you're just talking about adding a list of the spells by level at the end.Alphabetical and then by level.
I personally prefer a top level sort by level and a second level alphabetical sort, as it fits more closely with how I'd actually use the book as a player. You know you get to pick X new spells on a level up and you just got access to spell level Y, so let's flip to the section of the book with Y level spells and read until we find the spells we want to add.