Help With Cross Training

[Deleted User]

Hi, i've noticed that a few of you are into cross training and triathalons. A triathalon is my ultimate aim but am struggling with my bike sessions. I feel when I go out on my bike that I dont get as much out (CV-wise) of it as I do when I run or swim. I also dont think i'm a particularly energy efficient cyclist.
Can anyone suggest a good book/website or tips that can help me with my beginners angst?
flea