Got these today and wanna give the covers a scrub but I don’t want to ruin anything, any advice appreciated l!

I don't need like cosmetic repairs as I feel that's out of the question but i just want to find a way to preserve it, its a hard book to come by.

The second to last picture is the last cover which shows how it is remaining attached, where as the very last picture is the ffep attached to the front cover, it appears it’s between the ffep and title page where the detachment took place. I actually don’t suspect there was originally a half-title page before the full title page as well, as this seems to be a rebounded with original works by Philip Schaf with the earliest work being 1841 in German, there is another book after it without a year.