I want to learn how mainframe works on UNIX. Can you please provide me some reading/ reference material for the same. Its hard to find on internet. Also I don't know what level of knowledge is required on UNIX if I want to crack a interview in mainframe + UNIX.
I would think this would be your starting point: z/OS UNIX System Services
Check out the Library on the left for access to the various manuals.
And you must be really bad at using Google. Googling z/os unix returned 1,610,000 hits -- the very first of which is the z/OS UNIX System Services home page mentioned above.
The level of knowledge required will really depend upon what job you are going for. A system programmer will need a different skill set than an application programmer and both use different skills than someone writing scripts or Perl programs in z/OS Unix.
Are you saying you want to learn Unix to a sufficient level so that you can apply for jobs which require Mainframe and Unix?
For a Business Analyst? Something like "Unix for Dummies" (I hate referring to the titles, but read it as "Unix for Novices"). Check up on the languages/DBs available (although I presume you assume IBM flavour of Unix), but even that might be too far for a Business Analyst. The Business is more important than the System, isn't it?