Page 42 in the MGMAT SC guide:

"In most English sentences the subject precedes the verb. However, the GMAT occasionally attempts to confuse you by inverting this order and placing the subject after the verb."

I believe your example is such case. If you invert the subject, it actually makes more sense:

"Books made from 1994 to the present are included in the exhibition."

"Included in the exhibition," I believe is a modifier rather than the subject.

Uhuskey is correct here. Included in the exhibition is a modifier (past participle/non conjugated verb is how you can tell this type of modifier), and the subject is 'books'. It

Also, logically, 'included' cannot 'be' a book, but a book can be included.

Always ask yourself: What is the action; and then ask yourself: Who or what does this action? The answer to the second question ALWAYS gives you the subject, whether it is a single subject noun or a whole phrase.