Basic Workflow Question?

I have  a very simple workflow issue that I do not understand.

The Trigger Event is When Modified Record Saved. The Workflow Condition is when the Owner of the account field is set to Sarah Smith. [<Owner>]= "Sarah Smith". No other Conditions are active.

The Action that the workflow should do is to assign a book to the account.

When I modify an Account record FROM Kevin Ward TO Sarah Smith, nothing happens. I wrote the workflow with the assumptions that the workflow would at this point assign the book to the account. Do you know what I should do to have the assignment occur at this time?