Sony allow you to have six devices registered to one account. A device is a computer or a reader. So you and your husband could have a reader each, a computer each, and two slots for later. It is easy to de-register a device from the Sony EBL software, and if you have lost a device (I deleted a virtual machine for example) then a quick mail to Sony will eliminate that too. Finally, you can name each device under your shared Sony account "Angie's reader" for example.
The Sony store currently sells books that are mostly DRM protected and in a format called LRX (the protected version of LRF). I do not know of any way to remove the DRM. The store is switching to ePub, and will most likely use the ADEPT DRM that has been removed as explained elsewhere in these forums. It sounds as if you won't need to worry about that though given you can share an account across your devices.
As to bookstores, these forums are covered in recommendations! Personally, for classics I download from MobileRead itself, for books in copyright I prefer Books On Board in the US and Waterstones in the UK.