The domain of forge clergy Divinity of the channel: the blessing of the craftsman (XGtE, p.19) says:
From the second level, you can perform a one-hour ritual that elaborates a non-magical object that must include some metal. The creation is completed at the end of the hour, joining in an unoccupied space of your choice on a surface within 5 feet of you.
What you believe may be something that is not worth more than 100 gp.
So there are three requirements that allow duplicating an article:
- It is not magic
- It is less than 100gp
- It contains metal.
A spell book is included in the "Other Adventure Team" table (not magic).
The spell book, although it contains spells of a magical nature, is not considered, in itself, a magical item. That is, the rules for not explicitly saying that it is a magical element:
Essential to magicians, a spell book is a leather-bound tome with 100 vellum pages suitable for recording spells.
This is confirmed by an unofficial tweet of the main rules designer Jeremy Crawford:
A spell focus or a normal spell book is not a magic item. A magic is possible, like a magic cane.
Therefore, the spellbook is not magic (requirement number one: verify). It is also indicated in the table as having a value of 50 gp (requirement number two: verification).
Therefore, the third requirement is the only possible point of stagnation. The description of the spell book does not mention any metal in its construction. However, the rules do allow the DM to double them to allow this (Rule zero) and to say that there is a gold foil on the cover is a very reasonable narrative explanation for this.
Another option: the rule for an assistant to add new spells to spell book states:
For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents the material components you spend experimenting with the spell to master it, as well as the fine inks you need to register it.
Anyone who has seen an illuminated manuscript knows that highly decorated initials and margins are often made using aluminum foil and inks containing metallic pigments.
Therefore, if a DM allows a narrative explanation in this sense, a Force cleric could copy a spell book.