I've been thinking about preparing to some Unix Certification. I'm already SCJP, SCWCD and SCBCD. Although I don't have too much contact to Unix systems, I see that many companies require this knowledge.
Which one of these would be better for me?
Linux+ (CompTIA) Linux Certified Administrator (SAIR) Linux Certified Engineer (SAIR) Master Linux Certified Engineer (SAIR) Linux Professional Institute Certification Level I (LPI) Linux Professional Institute Certification Level II (LPI) Red Hat Certified Architect (Red Hat) Red Hat Certified Engineer (Red Hat) Red Hat Certified Technician (Red Hat) Certified Linux Engineer (Novell) Certified Linux Professional (Novell) IBM AIX and eServer pSeries (IBM) Sun Certified Network Administrator (Sun) Sun Certified System Administrator (Sun)
And, also, where could I find related material about it?
Some of the certifications that you listed down are for Linux and not Unix. If you are new to Linux/Unix, I think Linux+ is probably the easiest (and it's also vendor neutral). It can also give you a foundation to go into a more advanced certifications, e.g. RHCE which is a very popular certification for Linux.
Hope this helps.
SCJP 5.0, SCWCD 1.4, SCBCD 1.3, SCDJWS 1.4