In theory a good suggestion. In practise new members are easily turned off when the registration process is too complicated or when they are required to often to enter specific information (beside username, email, password).
Out of the blue I can think of two possible ways how to improve here:
1. Offer a drop-down box with the majority of current e-book devices including the specific models. Problem with this is that there are a lot of devices out there including all the OEM's to take into consideration, and that we'd be dealing with a static list that would require regular maintenance.
2. Have a friendly reminder notice where members are kindly asked to update the e-book field in their user cp.