The Collaborative International Dictionary of English v.0.48:

Reckoner \Reck"on*er\ (r[e^]k"'n*[~e]r), n.
   One who reckons or computes; also, a book of calculations,
   tables, etc., to assist in reckoning.
   [1913 Webster]

         Reckoners without their host must reckon twice.
                                                  --Camden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

reckoner
    n 1: an expert at calculation (or at operating calculating
         machines) [syn: calculator, reckoner, figurer,
         estimator, computer]
    2: a handbook of tables used to facilitate computation [syn:
       reckoner, ready reckoner]