The Collaborative International Dictionary of English v.0.48:

Vade mecum \Va`de me"cum\ [L., go with me.]
   A book or other thing that a person carries with him as a
   constant companion; a manual; a handbook.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vade mecum
    n 1: a concise reference book providing specific information
         about a subject or location [syn: handbook,
         enchiridion, vade mecum]