Detail or Details? Read on for more detail/details

I'm writing a newsletter and have been given the following text:

New stock has arrived & we're giving you the chance to grab it at 20% off for this weekend only! Read on for more details.

I'm unsure if the word details should be detail or not.

As other answerers have suggested, the more common course in situations like the one you raise is to use details (plural). But there are situations where "more detail" may reflect your meaning more accurately.

The key thing to recognize is that more can be used as a quantitative modifier (meaning "more numerous") or as a qualitative modifier (meaning "greater"). If you want to emphasize to your readers that they can gain additional specific pieces of information about your sale by reading on, the plural details meets your needs. And since you've already identified three details in the sentence "New stock has arrived & we're giving you the chance to grab it at 20% off for this weekend only!"—namely, (1) new stock has arrived; (2) you're selling it at 20% off; and (3) the sale will occur this weekend only—any subsequent details you provide will certainly be "more."

On the other hand, if you want to indicate that the additional content you're inviting readers to "read on" about is a more granular version of what you've already said, "more detail" would be a perfectly reasonable choice. For instance, here is a block of text that presents in more detail a description of the sale described initially by the sentence "New stock has arrived & we're giving you the chance to grab it at 20% off for this weekend only!":

We've replenished our aging stock of Furbies with 45 new models, direct from the Furby factory. Every new model will be on sale for 20% off the manufacturer's suggested retail price for all walk-in customers (limit: two Furbies per customer). But don't delay: This sale will take place on Saturday, September 21, and Sunday, September 22, from 10 a.m. to 6:00 p.m. only!

This text block provides more detail (that is, information at a finer level of precision) about the sale than your original sentence did, so it wouldn't be wrong to introduce it by saying "Read on for more detail." Of course, in another sense, the greater detail that you've provided takes the form of additional specifics (relating to product source, number of new products, buying limits, dates of the weekend days, and shopping hours), so "Read on for more details" would be a thoroughly defensible choice here, too.

"detail" is both singular and plural for particulars

But if you would replace "detail" with "features", "particulars", "specifications" they would all be plural so "details" probably reads better

The first is singular and the second is plural:

singular

There is one detail you missed: the butler has an alibi.

plural

He told me about his mishap, but I have forgotten the details.

here the reference said details would be appropriate for you.

"Details" is correct, because you've already been provided with one or more details.

New stock has arrived & we're giving you the chance to grab it at 20% off for this weekend only!

These are the first two details provided. Therefore, any further information would be "more details".

See the second definition of "detail" here: http://www.merriam-webster.com/dictionary/detail