The Archivist;- from where else can he/she learn new spells?

[Dwimmerlied]

One reason for the holy writings to exist is that giving divine casters immediate access to every spell in every supplement may seem extreme to some people. Some play that anything beyond core must be discovered in some way. They may also make some spells more rare than others. That means the only way to discover these spells is from other casters or from discovered writings. They could be from a church off the beaten path or an ancient tomb.

I've been contemplating this very thing. In my mind, this could only add to the game because you could introduce access to new spells in treasure hoards and such. All told, there are a hell of a lot of cleric spells.....