Some choices for Quantitative Methods are:
- Business Statistics Demystified by Kemp and Kemp.
- Business Statistics by Downing and Clark.
- Quantitative Methods: for Business, Management and Finance by Louise Swift and Sally Piff
- Statistics for Management and Economics 8e by Keller.
Regarding R:
- John Verzani. Using R for Introductory Statistics. Chapman & Hall/CRC
- Alain F. Zuur, Elena N. Ieno, and Erik Meesters. A Beginner's Guide to R.
Depending on your specific needs, you can look for more appropriate books here
Quantitative methods are research techniques that are used to gather quantitative data. If you mean statistics, of course you'll need it. It's what you'll use to analyze and present the results of these methods. Data mining is not exactly within the scope of quantitative methods, but it is correlated.