Current This is harder than it looks. If you are looking for recent material then you need to make two checks on your information. While magazines and journals will generally be more current than books, on certain hot topics even a book will get rushed into print. Look at the items in the footnotes and bibliography. Valid Most people forget to consider this point.
Libraries try to collect a wide variety of material for a wide variety of uses.
If you are doing serious research then you need to choose the most authoritative sources available in the library so consider two points. In some magazines the same author who writes this week on AIDS wrote last week on lawn mowers. Some publications are less reliable, authoritative, and accurate than others. As one extreme, just think of the supermarket tabloids. While they may have several stories on UFO's they are not likely to be your primary source for a report on extraterrestrial life.