Can anyone suggest reference material on mathematics applied to biology, in particular the study of the behavior of say simple unicellular organisms or cells? Ideally the level of complexity should be aimed at a competent undergrad. Thank you.
Added: Also, I am wondering if there are any areas of biology where more abstract branches of mathematics is/can potentially be used -- that would be very interesting. By abstract I mean not just crunching through scores of differential equations, but using say geometry, group theory and whatnot.