Studying machine learning algorithms: depth of understanding vs. number of algorithms
I would recommend limiting yourself to a few tried and trusted algorithms. I would not recommend Elements of statistical learning ( as a first book).  It is too theoretical, aimed  at graduate students, with exercises asking how to prove X or Y...  I think ISL is more appropriate, with more practical advice ( in any case both books are free as pdf downloads).

Besides statistics, I would make sure you are comfortable with experimental design/AB tests, and with Business Intelligence/Visualisation.