D. Y. Bardin et al., Z. Phys. C44 (1989) 493. (Scanned paper)
D. Y. Bardin et al., Comput. Phys. Commun. 59 (1990) 303–312. (KEK scanned document:)
D. Y. Bardin et al., Nucl. Phys. B351 (1991) 1–48. (Eprint hep-ph/9801208)
D. Y. Bardin et al., Phys. Lett. B255 (1991) 290–296. (Eprint hep-ph/9801209)
D. Y. Bardin et al., ZFITTER: An Analytical program for fermion pair production in e+e− annihilation, Eprint arXiv:hep-ph/9412201, 1992.
D. Y. Bardin et al., Comput. Phys. Commun. 133 (2001) 229–395. (Eprint hep-ph/9908433)
A. B. Arbuzov et al., Comput.Phys.Commun. 174 (2006) 728-758. (Eprint hep-ph/0507146)
   
  
A short historical overwiev, "Zfitter - past, present, future", was given at Loops and Legs, 2012 and published in arXiv:1302.1395