Lookup to another list using current item ID

xhn123456 

I do this all the time. Give the users read only access to List B. Create a look up in List A and use whatever field you need for the users to choose from as the drop down. Usually it's the Title field (or whatever you've called it) but it could be any form of text field. You can use a calculated field if the output is text. I use this frequently to concatenate multiple fields into one drop down, like "Customer Name - Project Number" as a form of data validation where any given customer might have multiple projects. In these cases, I usually bring both values into List A as separate fields.

In List A, user can select "Customer Name - Project Number" in the form - default form works fine - but in the list columns, they see Customer Name and Project Number as separate fields. Voila!