One of the volunteers' tasks is to tie up books whose spines or boards have detached or are detaching. This keeps the book together on the shelves, in its correct shape, until such time as we can box the whole book in acid-free boxes.
We use black tape as we think it looks better on the books than the usual natural colour tape, which may get dirty or discolour over time.
Who knew that book tying was so important? We had much fun practising this during our training with the conservator; try it yourself and you'll see how difficult it is to get the bows lying along the spine, which is the indication of a proper granny knot! Some of us are apparently natural granny knot tie-ers, some have to really think about it and do these instead of reef knots! (Natural granny knot tie-ers apparently have to keep tying up their shoe-laces...).