Rule #1 when writing your newsletter is this: focus on your reader, not on yourself.
After all, readers are interested in themselves, not in you.
So when companies fill a newsletter with how they've just got ISO 9001 or been awarded the Mayor's Prize for Human Resources Development, they're asking for their words to be thrown in the trash.
It's much better to write about stuff that will make your readers' lives better.
As always, Seth Godin says it better than me. Read Seth's post on Empathy and substitute "blog" and "book" for "newsletter". The same rules apply.
For more tips on creating newsletters, see the Elements of a Successful Newsletter section of this blog.