You need to store the Authers as a Child table to the Book Table,
These Videos are very helpful:
Relationships
Access Relationship Seminar

Combo Box is not ideal for this purpose, use List Box or Sub form instead:
Subforms

This needs to be set up correctly at the table-design level before you can do anything efficiently in forms.  Suppose your books table and authors table look like this:

BookID     BookTitle
1          Access 97 Developer's Handbook
2          A Brief Welcome to the Universe

AuthorID     AuthorName
1            Paul Litwin
2            Ken Getz
3            Mike Gilbert
4            Michael A. Strauss
5            J. Richard Gott
6            Neil deGrasse Tyson

Then you can set up what is usually called a "junction table" that links the books to the authors:

BookID     AuthorID
1          1
1          2
1          3
2          4
2          5
2          6

The above means book 1 is written by authors 1, 2, and 3, and book 2 by authors 4, 5, and 6.

When books are sorted by author, the author who is listed first is used for sorting.  At least that is how libraries do it.  So you need to keep this in mind if you do sorting in your database.  As the above picture shows, "Access 97" would be sorted under "Litwin, Paul," and the other book under "Tyson, Neil deGrasse."