The importance of good copywriting and appreciating user reading behaviour can be overlooked in the process of interface design. These articles from Baymard Institute make some good (and well referenced) points on headline writing, line length and help text.
A great article on writing headlines draws attention to context, considering Nielsen's advice which can lead to quite dull titles through to the (for me at least) really irritating teaser style: "7 really interesting things that you didn't know you didn't know about...". I think their conclusion is well balanced and an approach I'll be trying to adopt more frequently in email subjects.
Copywriting: How to Write Useful (Yet Intriguing) Headlines - article by Christian Holst
We all know that long line lengths cause problems for readability, but how long is long? And what about short line lengths? How short is too short? Here, the author pulls together recommendations and concludes that somewhere between 50 and 75 characters is the length to go for.
Readability: the Optimal Line Length - article by Christian Holst
Presenting help text in a manner that assists those that need it, and doesn't interfere with those that don't is a challenge. Here, Christian Holst considers 3 excellent examples (Google, Amazon and 37 Signals) which work really well in their contexts of use.
3 Examples of Inline Help - article by Christian Holst