Entity reference usage with coding

I have the following use case flow.
Step 1 User visits the site select a rental house (starting the site with only 1).
Step 2 After the house is selected a menu item must become visible with the option to book the house and to enter the weeks to stay at the house. Two dates one of the arrival and one for the departure week.

I have the following content type:

• rental house (text, pictures)
• booking (all personal details, bank account city etc.) booking has an entity reference field with the rental house
• weeks with weeknr, price and status of the week (such as booked)

After the hook save action of the booking the relation with the rental house must be made. With only 1 house it can be omitted of course. So after the hook_save action of contenttype booking some magic must be done but how? I can't find good example code. To fill in the field by the user is a bit ugly since he already selected the house in step 1.

Thanks in advance
John