I'm going to provide support for a more moderate and, I believe, more accurate answer.
There's a template idiom in the form "hit the noun" such as "hit the road" (something someone says before they leave in one's car for a trip or before going home late at night), hit the weights (go workout with weights), and "hit the books* (study hard, especially for a test). The OP's sentence is a part of a dialogue that must have been something like this:
Son: Dad, I know I have a test in Chemistry tomorrow, but I want to go hit the weights with my friend. (I want to go weightlifting.)
Father (Sentence 1): The only thing that I want you to do right now is hit-the-books.
Father (Sentence 2): The only thing that I want you to hit right now (2.a) is (2.b) are the books.
The father is using word-play on the son's use of "hit the weights" and moved "hit" to replace the word "do". The question is whether or not the shift requires a change in the verb from "is" (agreeing with singular "thing") to "are" (agreeing with the plural "books").
The prescriptive (self-proclaimed "correct" grammars including undergraduate, EFL/ESL, and proffessional style guides) answer seems clear: the subject ("thing") is singular, so the verb ("is") should be singular to match. Also, maintaining the original morphosyntactic structure helps to guide the listener to the salient part of the wordplay, which is the shift of the word hit outside of its idiomatic position.
In my opinion, the most persuasive argument for maintaining the singular verb ("is") is to help guide the listener in understanding the wordplay. This sentence is about the idiom "hit the books", and ambiguous grammatical issues are secondary to wordplay impact. Consider the following joke:
- Hyperinflation: Two pennies saved is a penny earned. (Louis Stein aka CoolHandLouis)
Using "are" is also funny, but weakens the reference to the famous adage, "A penny saved is a penny earned"--Benjamin Franklin.)
But the descriptivist ("linguistic, actual speech studies") view should not be totally discounted in this case. After all, if subject-verb agreement was the last word on this matter, then why would this sentence be so contentious--even for native speakers? The verb-matching-subject rule is not absolute when one accounts for some descriptive linguistics. (See Difference between prescriptive and descriptive grammar.)
Let's start with the reduced form of OP's sentence:
- The thing is the books. (Reduced form of Sentence 2.a)
Fundamentally, Sentence 2.a (the so-called "correct" answer), is grammatically defective. Recall there is another grammatical rule: the subject and subject's complement should match in number. The reduced sentence makes the disagreement between the subject's and complement's plurality obvious. What is one to do? Language is linear and we know "The thing is X" is better than "The thing are X", so we go with the former, which is the subject-verb agreement rule. But what is "the books" then? It must be thought of as a collective noun, even if it doesn't look or feel like one. Forcing something to be a collective noun is related to the idea of notional agreement.
But adding a few more words helps the feel of the sentence:
- The thing I want is the books.
Above, the expanded semantics helps eliminate some of the cognitive dissonance while retaining the natural grammatical agreement of subject and verb. And as we add even more, the cognitive dissonance drops to such a low level that the plural ("are") form can sound fine to many people. Consider the following:
...the only thing left for me to read are the books on my shelf.
(Actual comment found at http://www.thesepaperhearts.com/2014/05/discussion-reading-preferences-new-old-books.)
This phenomenon is called proximity concord (aka proximity agreement) and is found in actual spoken and written English. Here's another one:
#The Only Thing Better Than Waffles Are Waffles On A Stick
(Cute article title at "Oh Gizmo!")
Similar to my "pennies saved" joke, this cute article title was almost surely chosen intentionally for effect (in this case, the euphonious effect using proximal concord).
You'll find proximal concord most often in quoted speech, as seen with Martin Cuddihy, ABC's Africa correspondent in, "...about the only thing to grow are acacia trees.". But it also sneaks into more formal areas. Ross Werland, editor of Chicago Tribune Travel wrote, "The only thing I missed are 360-degree wheels...". And Frank Lloyd Wright, the famous American architect, said, "The only thing wrong with architecture are the architects."
Proximity agreement occurs when there's a plausible shift in plurality and the phrasing distances a copular verb (such as "to be") from the grammatical subject, resulting in a more natural sounding verb when it matches the closer, logical subject. This phenomenon is more pronounced as the verb gets further from the grammatical subject, the verb gets closer to the subject complement, and any intermediary phrases include plural phrases or semantics.
Now let's now revisit the OP's original sentence:
Sentence 2: The only thing that I want you to hit right now **(2.a) is (2.b) are ** the books.
(The original sentence in question.)
Sentence 2 contains a jumble of forces pulling it this way and that. Grammatical agreement and a notionally plausible, collective-noun interpretation of "the books" pull it toward the singular form (1a). But the plurality of "books" and the proximity agreement effect pull it toward the plural form (1b). These forces result in the varying preferences among native English speakers.
Such a spoken statement, in contexts real or fictional, would virtually never be subjected to grammatical correction. Compare to the following sentence, which is in the formal register:
In regards to the library budget, the only thing that matters at this time
areis the remaining books.
A good copy editor would have no compunction in correcting such a prescriptively incorrect subject-verb agreement, should it be included in an opinion piece, for example.1 But that's professionalism; neither would a good copy editor dismiss Sentence 2.b as being wholly "incorrect" or some type of grammatical blasphemy; it's clearly in an informal, spoken register in which proximity agreement is not uncommon.
In conclusion, due to the ambiguity and subtlety of this issue, either form in this case is OK in typical, everyday communication. The sentence will read/sound fine to many native English speakers using either the plural or the singular. Unlike some other grammatical mistakes, the "incorrect" plural form is well-hidden and natural sounding (to many a non-grammarian), so there is no stigma-of-being-incorrect associated with it. Most people wouldn't even notice--or care if they did.2
1. In Copy Editing: Agreeing to Disagree (Oct-Nov 2011), Brenner proclaims, "We copyeditors are duty bound to fix proximity agreement." But actually my example sentence is somewhat forced. An editor may also reword the sentence to avoid the issue.
2. This is in contrast to something like "I ain't no doctor" in which a large percentage of people would notice the double negative, which may affect their opinion about the utterance or the person making the statement.