I can't even tell you how many times I've had this conversation:
Designer: If we want this to fit on one page, we'll need to cut some of the text.
Librarian: That reminds me, I thought of a few things to add!
Librarians are "Information Professionals." Connecting people to the information they need is part of a library's core mission; it's not surprising that librarians always want to add one more link to the website, one more sentence to the description, one more book to the list.
This is not usually a good idea. Why? Well, in order to answer that, let's talk about the function of whatever-it-is I'm producing for you.
If the purpose of the piece is to advertise something - say, a weekly "teen gaming" activity - then it's important to concentrate on the basics: let people know when and where, and make it look appealing. Now, information professionals usually want to list the games we have available, or include some information about how gaming increases literacy, and maybe some links to some articles. Soon, the flyer is covered with tiny, tiny text. When-and-where is hidden in a mass of other information, and the appeal has been lost. No teen would ever pick it up this document, because it looks like homework - an academic paper, complete with footnotes, crammed onto a half-sheet of paper. Advertising pieces can only EFFECTIVELY convey a bare minimun of information, so choose carefully. Decide in advance what the piece needs to convey, and then limit yourself to those necessities.
But what about informational documents? Actually, the rules remain true. It's equally important to prioritize your content - decide what you must include and what you'd like to include if there's space. You don't want to risk leaving out or obscuring something truly important in favor of something that's only tangentially relevent.
It's also important to remember, when you're making a list of selected resources, that "selected" is a key word here. People don't want to wade through every book in the business section; that's why they need a librarian's help in the first place. By the same token, they don't want to wade through a list of every book in the business section. They need us to narrow our focus so that we can help them narrow theirs.
I hope my librarian co-workers understand this - that when I ask them to trim lists and cut text, I'm not trying to limit the flow of information for the sake of "making things pretty." It's all about function and usability.