For an undergraduate and for those weak in Math in their masters can do their study with the following text books
1) Fundamentals Of Mathematical Economics BY Alpha C Chiang
2) Introduction To Mathematical Economics BY Edward T Dowling IN Schaum's series
They both form a very good combination. Schaum's series is amazing.I just got my personal copy of the 2nd book.
I am doing my reading on Mathematical Economics now.Very soon i will come up with a blog ranking the top 5 universities/institutions in UK, USA and India in economics