Adam Smith’s two major books are to democratic capitalism

[RonPurewal]

i mean... it's like writing "Chicago is similar to Detroit is to Milwaukee."

...not a sentence.
honestly, this should be perfectly clear. there's no way to write a valid english sentence whose structure is anything like this.

[JbhB682]

Hi - in the idiom : X is to Y what W is to Z : All four of them should be nouns (At least in the examples provided by Ron)

But in this case

q1) seems like we have clauses in some cases ? Is that acceptable ?

q2) in this case, the idiom is structured different : X that are to Y what W is to Z...How is this acceptable ?

Adam Smith wrote two major books that are to democratic capitalism what Marx’s Das Kapital is to socialism.

X : Adam Smith wrote two major books
Y : democratic capitalism
W : Kapital
Z : socialism.

[Sage Pearce-Higgins]

Take care to note that the idiom in its simplest form is X is to Y what W is to Z. You're right that we're going to find nouns here, as we need nouns to be the subject of the verbs 'is', plus there's a kind of parallel / comparison structure going on.

However, this case is complicated the fact that it's in a modifier. The author could have written 'Adam's Smith's books are to democratic capitalism what Marx’s Das Kapital is to socialism'. I'm sure you'd agree that we have four nouns here. However, instead we have a noun modifier introduced with 'that'. Since its a noun modifier, it's giving information about 'books', not the whole clause.

Think how this works in a simpler construction. If I write: 'I have a dog that is bigger than your dog' I'm clearly comparing the two dogs, not the clause with a dog.