Tenured Radical (Claire B. Potter) has a nice post up titled These Things I Know: Applying for Tenure-Track Jobs. Although targeted at historians, it has good advice which can be fitted to other disciplines including librarians. However, I question the last point made.
Potter wrote, "Oh yes -- and for god's sake, use letterhead. Your department letterhead, the letterhead from the school where you are teaching a course for $2300 -- whatever letterhead you can get your mitts on. A job application on blank paper, in a blank envelope, can't help but remind its recipient of a chain letter or some other degraded piece of mail."
I have served on numerous library search committees at three universities. Many of these were for tenure-track jobs. It is very rare that an applicant applies using the letterhead of the institution they work for presently. When it happens, comments made by search committee members have not been kind. Is applying for a job at another institution part of the official business of your current job? If not, it would appear that the applicant was misusing the resources of their current employer. Search committee members do normally read your c.v. and they can figure out where you work without the letterhead.
It might just be a history thing. I have never been on a history search committee before. If that field operates in that way, OK. I am sure however that applicants from other fields that do not operate in that manner will find this post and may make a mistake if the follow that piece of advice.
Not that it matters as much anymore. The electronic application systems many HR departments are using now do not allow for mailing in applications anyway. It all has to be done online.