[e2e4]

I went to a shop and bought a book there.

It was written by J.J.

I (had) read all the books he had written. (it means I read all the books he had written but not this new one)

These books had been (were) of great interest to me. (riverkid, thanks for the of great interest)

I had been enjoying them very much.

Here and now I am mostly interested to know is it possible to use were instead of had been in the fourth sentence and in the context of the story.