If the body of the book is intact, and all you need is the cover reattached, I will:
a. Add new flyleaf.
b. Reinforce the spine with binding gauze.
c. Replace or add headbands if needed.
d. Glue on cover. Create new spine, if needed, using color coordinated book cloth.
Although I do not specialize in returning old and rare books to their original condition, I am able to reattach old covers by making a new spine and only covering up a bit of the original cover. This allows a book to be read again when otherwise it would sit on a shelf unread.
NICODEMUS This 1936 book was really falling apart. I re-glued the pages and reattached the cover.
WINKIE BUNNY TAIL This 1936 book was also falling apart. I was able to re-sew the pages as they were originally done, then reattached the cover.
THE LIFE OF REVEREND JOHN WESLEY
This 1831 book was still fairly tightly bound. I re-glued the few loose pages then reattached the cover.