The email or the sales letter has the purpose of grabbing attention and exciting the reader - making the reader think: I want this, I want to know more about this.
The brochure or the landing page on the website carries the details, the technical data, the features, the specifics.
This separation of the two elements is normally really important although not always essential - for example, if you are simply selling a report you can probably say what you need to say about the benefits of the report on an email and leave it at that.
But if you have something more complex than a report, or if you want to incorporate options, possibilities, what others have said about the product, etc, you need something separate. Normally a leaflet or a special landing page on your website.
Because the email normally leads onto a landing page, and because a sales letter leads onto a brochure or leaflet, it is vital to keep both in mind when writing.
In an ideal world this means writing both the email and the landing page at the same time, but if this is not possible, always have the landing page in front of you while you are creating the email – or vice versa.