Which one is correct?
Sales of the spy novel reached their zenith in the 1960s.
Sales of the spy novel reached its zenith in the 1960s.
They (sales) reached their…
Sales of the spy novel reached its zenith in the 1960s- is the right one.
Though ‘sales’ looks like plural, I am afraid one has to use only the singular verb.
You’re wrong, I’m afraid, Q09.
This is correct:
b sales [/b](of the spy novel) reached their zenith.
Which would you use here, and why?
Reaching their zenith in June this year, sales (of the novel) have now started to fall.
Reaching its zenith in June this year, sales (of the novel) has now started to fall.
?*Reaching its zenith in June this year, the sale of the novel has now started to fall.
Reaching its zenith in June this year, the sale of the Nova model has now started to fall.
Yeah , now even I think their will be correct.
After all we say: ‘Sales are increasing’ and NOT ‘Sales is increasing’
Exactly. 
Sale report or sales report, which one is right?
Sales here refers to the totality or effect of transactions at various places and in this context I feel treating that as singular is right.
‘Sales’ as noun generally refers to the effect of selling things and not to different transactions. The same word,without the ‘s’ -‘Sale’- has got a different meaning.
So ‘sales’ is not the plural of ‘Sale’ even like ‘news’ is not the plural of ‘new.’
I agree with Molly. “Sales” as a noun indicates plural. Nanucbe may be thinking of “sales” as an adjective (like sales report) but then it wouldn’t be used by itself.
You might like to write your own grammar book.
Until then:
sales
plural noun
the number of items sold:
Sales this year exceeded the total for the two previous years.
See also telesales.
sales
group noun [U]
the department of a company that organizes and does the selling of the company’s products or services:
He works in Sales.
the sales department/manager
[b]
[/b]
The second comment is very pertinent.
Webster: Adjective: of, relating to, or used in selling
The free dictionary: Noun: 7. sales
a. Activities involved in selling goods or services.
b. Gross receipts.
The two sources differ on the parts of speech to which the word belongs.
But I still feel that this should take only the singular verb.
Nouns can also be used as adjectives but plural nouns are not used as adjectives.
Only the singular form is used.
A five dollar note or a five dollars note?
The use of ‘Sales’ as adj. also indicates that the word ‘Sales’ is treated as singular and not as plural.
English is very dynamic. I wouldn’t use sales as singular myself in this case but I would hesitate to call it wrong. Like you can say “Five dollars is not a huge sum of money”…Five dollars is viewed as a singular sum in this case…so why not sales as singular?
Thanks for the compliance.
Even here you have used only the singular, though you have not enclosed them within single quotation marks. There are certain peculiarities in English that baffle even the Oxford English professors.