“Truest of friends” vs. “truest friends”
The following are two authentic examples from Google Books:

Both women experience loss, from men to dreams, but through it all they remain the truest of friends for 50 amazing years.

Loyalty makes the wolf a good member of the pack, the dog the truest of friends, the savage a good member of the band, the tribe.

I'm wondering what role the preposition 'of' plays in these examples. Why not just say "truest friends" instead?

Are these two constructions interchangeable? Any contextual nuances implied?

Question

“Truest of friends” vs. “truest friends”
The following are two authentic examples from Google Books:

Both women experience loss, from men to dreams, but through it all they remain the truest of friends for 50 amazing years.

Loyalty makes the wolf a good member of the pack, the dog the truest of friends, the savage a good member of the band, the tribe.

I'm wondering what role the preposition 'of' plays in these examples. Why not just say "truest friends" instead?

Are these two constructions interchangeable? Any contextual nuances implied?

Answer

When you say "they are the truest friends", my first thought is "they are the truest friends of whom?"
You should just say "They are true friends." ('of each other' is understood)

When you say the "they are the truest of friends", you are talking about those two people being very true friends of each other.

Answer

Well to be clear, you can have only one truest friend so "truest friends" does not work.

"truest friend" or "truest of friends" is the same.  Though both are un common phrases, best offered on the eve of battle :-)