The Forge Domain cleric's Channel Divinity: Artisan's Blessing (XGtE, p. 19) states:
Starting at 2nd level, you can conduct an hour-long ritual that crafts a nonmagical item that must include some metal. The creation is completed at the end of the hour, coalescing in an unoccupied space of your choice on a surface within 5 feet of you.
The thing you create can be something that is worth no more than 100 gp.
So there are three requirements that allow an item to be duplicated:
- It is not magical
- It is less than 100gp
- It contains metal.
A spellbook is listed among the table of (nonmagical) "Other Adventuring Gear".
The spellbook, though it holds spells which are magical in nature, is not, in and of itself, considered a magical items. That is, the rules to not explicitly say it is a magic item:
Essential for wizards, a spellbook is a leather-bound tome with 100 blank vellum pages suitable for recording spells.
This is confirmed by an unofficial tweet of lead rules designer Jeremy Crawford:
A normal spellbook or spellcasting focus is not a magic item. A magic one is possible, such as a magic staff.
Therefore, the spellbook is not magical (requirement number one: check). It is also listed on the table as having value of 50gp (requirement number two: check).
Therefore, the third requirement is the only possible sticking point. The description of the spellbook does not cite any metal in its construction. However, the rules do allow for the DM to bend the rules to allow this (Rule zero) and saying that there is gold foiling on the cover is a very reasonable narrative explanation for this.
Another option: The rule for a wizard to add new spells to a spellbook states:
For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.
Anyone who has seen an illuminated manuscript knows that the highly decorated initials and marginalia are often made using both foil and inks containing metal pigments.
Therefore, if a DM allows a narrative explanation along these lines, a Force Cleric could indeed copy a spellbook.