Apple has a certification program analogous to Microsoft's MCSE (Microsoft Certified Systems Engineer) certification. They have classroom training at authorized training centers, but they are offered in far fewer cities and less often than those for Microsoft or Cisco. If your employer is willing to pay for it, it would be a good idea to pursue this.
I hold three Apple certifications (see my Ask Different profile here on this site) although none of mine are in network administration.
You might want to pursue the Apple Certified Technical Coordinater certification. If not, at least look at the training materials and third-party books that cover preparation for the Apple OS X Server Essentials 10.7 Exam (9L0-515), or the previous exam for 10.6, which is still available.
Look at Apple's Training and Certification web site to learn about their program.
In the meantime, I would steer you to the Apple-authorized training books from Peachpit Press.