、。
Books are to the mind what food is to the body.
Literal
Book [attributive-の] mind [toward-に対する] relationship [topic-は], food [attributive-の] body [toward-に対する] relationship [to-に] equal.
A philosophical analogical statement. ~に対する ('regarding, toward, against,' the attributive form of ~に対して) modifies both 関係 ('relationship') phrases in parallel. 等しい ('equal, equivalent') takes its comparandum with に. A formal, essayistic register — the kind of sentence found in a literary or philosophical essay.