It's an object. Look at it this way:
Mary Carson wrote a book.
"Book" is undeniably the object of the sentence, of which Mary Carson is the subject. Now, subject complements are adjectives that relate to the subject directly with a linking verb, for example "I am hungry." (Here is a good description of subject complements.) Look at this example:
Mary felt sick.
"Mary" is the subject, "sick" is the complement. Note that sick is an adjective. Now, look at this:
Mary felt her forehead.
Here, "forehead", which is a noun, is what is felt. So, forehead is the object.
[Edit] I'm going to quote StoneyB's explanation of the difference between complements and objects from his comment below, and delete mine. StoneyB's explanation is precisely correct, and mine was oversimplified enough to be inaccurate (while objects are always nouns, and complements are often adjectives, to say "complements are adjectives" as I did is incorrect).
Predicate complements are arguments of copular verbs: be, seem, become, &c. Objects are arguments of transitive verbs: write, . A nominal predicate complement identifies the subject; an adjectival predicate complement describes the subject; an object is in some sense acted upon by the subject (in the active voice).