Lets say: I have a book and this book has text in it, but: if i want to have 2 books, i just want the same actor class with changed text.
So how can i create changeable variables in an actor so i dont have to copy the blueprint and change the hole thing?
I know that it has something to do with structs and set member… 
pls halp
You could do this in many different ways. One way would be create an array of text data within the book actor with each member containing the different texts that will be used, then wherever the text is used, use an array ‘get’ to retrieve the desired text passing in an exposed (which can be changed in the editor) int variable as an argument to the index of the array ‘get’.
See the documentation on arrays if you are unclear.
Solved:
Create a Structure Blueprint
Import it as variable “structure” into your blueprint
Create all Variables u wish to edit in both Structure and Actorblueprint
Go to the Construction Tab of your reading Actor
Search for a Node called: Set members
Add the variables
FUN
Arrays have nothing to do with such a simple thing 
Actually this sounds like a job best suited for inheritance. Have a parent class called book with all they key functions of what you need the book to do. Then have each child be a different book with different text, model, texture, etc.