Current Customer / Sessions

I'm having trouble finding information about customer sessions. If I wanted to code a page where the user could click a button and get an e-mail listing how many products they have bought (or any arbitrary information from their personal account/database table) would I locate the current user by looking into the session information? If not, how might I be able to access the current customer to do mySQL look-ups on their account information?

The customers_id is references as:
$_SESSION['customer_id']

when the customer is logged in ...

So just to be clear- also I apologize if this sounds stupid, as I'm not very familiar with web dev- you can access the customer id from that reference from any php file in the server's zen cart folder. i.e. i could just type

echo $_SESSION['customer_id']

as the body for a page linked to the sidebar and it would print the customer's id? (although printing info is not necessarily my goal)

Yes, the current logged in customer can be checked with the:
$_SESSION['customer_id']

and when 0, then the customer is not logged in ...