I think you can get many excellent things from book plans and on line coaching - but they aren't going to be able to tell you if you are mashing the pedals, or pedaling in too low a gear.

I have both followed books and had an actual coach this year -- I just started biking in March. I do find the book plans helpful - I plan on trying to map out the Friel plan for next year. (Friel is pretty complex, I will get the book that was recommended here - Friel needs an interpreter)

This spring I did a few one-on-one sessions with a live coach, which were very very helpful - my coach Felix is what really made me into a biker. He is very old school - no heart monitor or even cadence meter. What he taught me was proper bike handling ie : pedaling style, when to stand, what gears to use when , position on the bike and bike fit. One of the most important things I've learned from Felix is how to ride in a group - something you just can't learn from a book. I will continue to ride with his group regularly.