Start by going to your Library.

To the left, you will find the Books available in Magma. You can add these books to your list -> My list, to the far right.

To add a book to your list, click the Add+ -button. The book will then be moved to your list.

You can also filter through the books here:

You can look through the material in a book when creating a new assignment and choosing the book.

You can remove books from your list as well. To do this, click the subtraction sign to the right of the book's name.

So, if you accidentally remove a book, or notice that a book is missing from your list, you will be able to find it to the left among the books - and add it again.