Phrasal adjective before/after noun. US/UK usage split?

Solution 1:

Given that this is responding to your footnote, rather than your main question, perhaps it should be a comment, but:

It seems to me that there's an elliptical "that are", making "hard to find" a relative clause.

As for the difference between "hard to find" and "vital to life":

In "vital to life", "to" is a preposition. The prepositional phrase "to life" modifies the adjective "vital", which in turn modifies "compounds". The compounds are vital. What are they vital to? They are vital to life.

In "hard to find", "to find" is an infinitive; "to" is a particle attached to "find". "Hard" modifies this infinitive. "Hard" doesn't modify "books", it modifies "to find", and then that whole phrase "hard to find" modifies "books". Try applying the last three sentences of the previous paragraph here. "The books are hard. What are they hard to? They are hard to find." That really doesn't work.

Some adjectives can take prepositional phrases; they modify one phrase, but take another phrase as an object. When that happens, the adjective is often moved after the phrase that it modifies so that it can be next to both the phrase that it modifies and its object:

"tools useful in mining" rather than "useful tools in mining"

"toys interesting to babies" rather than "interesting toys to babies"

"jokes inappropriate for work" rather than "inappropriate jokes for work"