
I recommend James Murray mathematical biology book for nice examples of how ODEs can quickly give insights into complex phenomena.
Mathematical approaches differ greatly depending on the area of biology. I could recommend a good review if I knew your favorite area.
Then also textbooks with names like "Mathematical Models in Biology" or "Mathematical Biology" should have chapters on population growth, a two-species predator-prey model, diseases and epidemiology, and sometimes also a chapter on chemical kinetics (these are all modelled with differential equations).
Fractal models can generate patterns reminiscent of some organic structures, particularly branching structures, but their importance in doing so is often overstated. Generally speaking, they don't capture the underlying biological processes giving rise to the structures; models that do (reaction-diffusion, optimal transport networks, models of specific genetic developmental programmes...) achieve more realism.
"The problem with a field such as fractal theory, which can be visually dramatic and practised without much background and sophistication, is that uninformed proselytising and inappropriate use can raise unrealistic expectations as to its relevance and applicability. ...Although chaos and fractal theory have been proposed by some as biological panaceas fortunately there are enough realists to counter this view and generally keep them in perspective." –– JD Murray, Mathematical Biology
I was intrigued by "Essential Mathematical Biology" http://www.amazon.com/reader/185233536X).
I usually pick a book based on amazon ratings. I also pour through HN comments and find it equally rated.(eg: "random walk thru walk street" on amazon vs the comments here at HN)