Here are some suggestions for MIGRATION sub-fields (as I use today under my custom's tags):

source

start writing date

end writing date

book title

type of process (author/editor/printer)

editure

editing date

editing place

comment (obs)

When starting dealing with the aspect of "books" you should also take into consideration the fact that "some" of your users do already have such fields under their custom's tags or under your occupation tags; therefore you should provide them with a conversion tool. For example, let them copy every already existent field of them to the correspondent new one (appearing under a specific EVENT).