Dictionaries

Some of the inherent problems with print versions of dictionaries are; lack of use (they grow more dust layers than the leaves on the fake plants), outdated too quickly for the cost (replace every 5 years?? they are only cheap enough to buy once they are already 2 years old, inappropriate reading level (is it for the intermediate grades or secondary, what if my school is K-7? or 8-12?), information is separate from the index (how do I share one index with 30 students in a class, and not find them off task?). Also, all of the information is presented in the print version on a level playing field. No one topic would draw a students attention unless it contained an interesting photo. 

Essential special dictionaries would include; a English-French dictionary (and other languages if a secondary school, such as German, Japanese or Spanish), a thesaurus and rhyming dictionary, topical dictionaries for vocabulary such as Harry Potter, skateboarding, and medieval terminology. 
As for telephone directories, they are cumbersome paper wasters. Often the numbers are difficult to find in the yellow pages, or not updated in the white pages. 
My guess at the teacher librarian's essential directory would be SLiP.