Not to undermine what Keef said, but it is not essential to have a thorough knowledge of MEL to get work in a studio (unless, of course, they specifically ask for it). Obviously having some knowledge of MEL would be advantageous, but having knowledge of other languages is also good, such as c/c++/python/perl,etc. From my experience within my company knowing MEL is not essential to becoming a rigger, as rigging does not just involving writing expressions/coding. Some people have been learning rigging with only basic knowledge of MEL, they just learn on the job.
In the end, its down to the requirements of the studio, and learning any programming language is always really helpful, especially MEL. Good luck with it.
p.s, a good book for you may be Complete Maya programming: an extensive guide to MEL and C++ API by David Gould .