a mag will give you all sorts of tips and advice but unless you have the fundamentals in place, these can confuse you enormously!!!
your best bet is to go out and ride the bike - maybe with experienced riders - and learn from that experience. to an extent the same with swimming although 1:1 lessons or working with a tri club swim session will give you more benefits on techniques.
but if you want a mag - something like 220 or Triathlon Plus are stacked with tips and hints of all types. on the bike side - Cycling Plus, Cycling Weekly are as good as the tri ones.