Item availability in row or another table
It's depend on what you want achieve. What your database will do? Do you want track who lend book and when? You can create table with transaction and based on return date set availability. Another question is how you want to implement this logic. It could be controlled by your front-end or you can create view, stored procedures or implement triggers. For simplicity you can add column IsAvailable as bit (1,0).
I don't now what database you are using.