Which webform hook and how use to set programmatically webform target email?
If you just want to customize the target email, webform do this quite easily.
Go to node/123/webform/emails (where 123 is you nid) and there you can configure the recipients.
The option "component value" allows you for example to set the email regarding a select list. Configure a select list for the subject, put these values in the option textarea:

contact@example.com|General inquiries
marketing@example.com|Marketing


Then when the user will use webform the email will be sent to the direction chosen in the subject list.

If you want something more advanced, have a look at my previous answer in "pass cck email value to webform block". This will help you to detect different webform (using hook_form_alter.