Parishes can often ill afford a Gospel Book with an ornate cover (there are still some on the market) so a book with a simple red and gold cover can be purchased and those parishes than can afford it can purchase ornate covers to place over them.
A little over ten years ago (when I worked there) my parish purchased a Book of the Gospels with an ornate cover. The cover started coming loose from the book after a couple of years (when I no longer worked there). In order that book wouldn’t be completely ruined, the book was retired and a slightly smaller Book of the Gospels with a less elaborate cover but a better binding was purchased. It might have been a multi volume set; I’m not sure.
Sometimes purchases are made for practical reasons. (I’m pretty sure my parish could afford a more ornate book if it was well made.)
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