I have a website where I post my photos and with every photo a "saying". When I manage my photos in Aperture I add the "Saying title" in the metadate "Byline Title" field and the saying in the metadata "Byline" field. So far so good. Now I want to make a book but when I add a "Metadata Box" I´m not able to pick the Byline Title or Byline in the "Set Metadata Format" option. Can this easily be fixed or what?

In a word, no. The choices given are the only ones available. Your only practical option at this point is to move or copy the metadata to one of the available choices before you import into Aperture. There are no capabilities inside Aperture for moving or copying metadata between fields automatically.

When laying out a book, metadata boxes can be added and linked to images. Having selected the Edit Layout mode and clicked on an image, the Add Metadata button (center) creates the box:

The metadata box is linked to the selected image and includes the data shown by the tag:

There is a drop-down menu that allows the contents to be changed:

Only this small selection of possible metadata can be displayed. The only way to get more data is to move it into the available fields.

But I have never been able to get anything to appear for the Title, Author, or Copyright selections. It's as though Apple has simply missed something out here.