In theory you can create a bundle product with one bundle item that has the downloadable book and the physical book as options.
But if you look at app/code/core/Mage/Downloadable/etc/config.xml
you will find the following under global/catalog/product/type
:
<!--<bundle>-->
<!--<allowed_selection_types>-->
<!--<downloadable/>-->
<!--</allowed_selection_types>-->
<!--</bundle>-->
<configurable>
<allow_product_types>
<downloadable/>
</allow_product_types>
</configurable>
It looks like downloadable products were once allowed but since Magento 1.6 this was deliberately commented out, so probably there was a problem with it (see also this question). You could try to reactivate this code nevertheless but be careful and expect bugs with that.
By the way, since this is not directly programming related, your question will be more welcome at magento.stackexchange.com