Library Database

I am trying to create a database for an elementary school library. I have downloaded the Library Manager, and while it is quite nice it is more complex than I need. There are two things I am trying to do that I can't figure out.
I would like to have a form where I could input an ID number assigned to a book that is in the database and have it populate the fields for the author and title for that book.

I am also having a problem with a formula field. I have field called "CheckOut" that is a date field. I also have a formula field called "DueDate" that I am trying to have auto fill with the CheckOut date + 7 days. Here is what I am using for formula:
CheckOut
(
type = date
)

DueDate
(
type = formula
value = (CheckOut + '7D')
)
The Due date field fills in but it has the same date as the checked out field. and when I go back to edit the Due Date field the code is missing and it only says CheckOut
What am I doing wrong?