CreateBooklet.cs

var files = Directory.EnumerateFiles(".").OrderBy(x => x).ToArray();

for (int i = 0; i < files.Length / 2; i++)
{
    var paper = i / 2 + 1;
    var side = i % 2 == 0 ? 1 : 2;
    var page1 = i;
    var page2 = files.Length - i - 1;

    File.Move(files[page1], $"{paper}-{side}-{(side == 1 ? 'a' : 'b')}-{page1 + 1}.jpg");
    File.Move(files[page2], $"{paper}-{side}-{(side == 1 ? 'b' : 'a')}-{page2 + 1}.jpg");
}