Understanding Deep Learning is a very approachable text that will get you 80% of the way there. Dive into Deep Learning is another. Both have free PDF version

Understanding Deep Learning is a very approachable text that will get you 80% of the way there. Dive into Deep Learning is another. Both have free PDF versions available. The math isn't difficult. The notation is a little foreign, and you have to take your time reading and rereading the equations.