How to handle ragged corners but especially ragged edges

One way is to treat the inner split edge like you would for the corners. Trim off those loose threads. Color the edges of the cloth using acrylic paint to match or whatever other color medium works for you and this book. (color pencil, archival markers) Get paste in between the layers and press until dry. You can color the board directly or color Japanese tissue to fill in the missing cloth. 

I hope this helps!

Huh. OK, tonight the separated corners and board edges will be addressed. I have purchased a small set of acrylic paints and will take their use under consideration. But I must say watching Sophia use Japanese tissue to "repair" the board edges was captivating. A very brief search of the Internet regarding Japanese tissue for various paper/book repair opportunities was, at best, overwhelming. Can the Japanese tissue offered on the Save You Books website serve this purpose? Can those tissues (white or natural) be colored with the acrylic paint before applying to board edges?

Yes, the Sekishu tissue is ideal for board edge touch ups and will take the acrylic paint well both before applying adhesive and afterwards. You will not have to overlap the edge much at all for this to work. Use the least possible for the best looking result.

And perhaps the last question for this project is the lettering on the front cover. The letters at the very top (Big Book Of) are "yellowish" but the basic underlying red has started to show through due to wear. Is this something I should even try to address? Even parts of the horse are starting to show a reddish tone through the black areas. Are there archival pens of varying colors one could use? I'd really rather keep the worn look than make it look garrish and artificial. Comments?

Will move on to coloring/tissue repair of edges