Why is poussière singular here?
Why is dust singular here?
It may be that poussiere is not a countable noun. Similar to 'sugar' or 'salt'
No, it's not an uncountable noun in French: les poussières is frequently used.
Why is 'être' used here and not 'avoir'? Wouldn't it be better with 'avoir'?
Why not "anciens"?
I don't think they are former books.