I have just finished watching the episode on videotape this morning, and have to say... *WOW*!
"Silence in the Library" has to be one of the best episodes of the current new season (the fourth, I think) of Doctor Who so far. Steven Moffat wrote another brilliant episode of the series here that plays on the basic fears that we all have, and the supporting cast, notably Alex Kingston (who I really like as an actress) was superb. I can't say that Catherine Tate's Donna Noble has been all that likeable as a companion in my opinion, but I much prefer her to Freema Agyeman's Martha Jones, for reasons that I think are interesting. Competent companions, at least overly competent ones, tend to be less enjoyable to watch in my opinion (with a few exceptions), because they don't bring that sensa wonda to them that the Jo Grant, Sarah Jane Smith, and Rose types do.
In any event, superb episode, and I'm looking forward to next week's story now. And to seeing what Moffat does when he takes over the series in 2010, of course.