I'm just a humble engineer. I've taken the standard slew of math courses everyone takes (Calc, diff eqs., linear algebra) and I enjoyed them but I want something more. The world of math is so vast that every time I try to branch out I get lost and give up. Do you guys have any suggestions for books/websites or topics that would help me advance past the standard. The problem I always encounter is that books are either too fruity with little rigour or they assume a lot of notation and background that I don't have. Any suggestions would be appreciated.