I have both books.
The ABC's is the best place to start as it actually breaks down all the processes of reloading into steps and gets very detailed on what to watch for and how to do it.
The lee book does the same, but not in as great of detail. The majority of the lee book pertains to using lee products (fine with me!) and load data. The ABC's has zero load data.
START WITH THE ABC'S. I promise, you'll thank me. I love my Modern reloading, I would absolutely get it second, as you'll understand more about what Mr. Lee is talking about because the ABCs spelled it out for you. Also, be careful and get the latest edition, some places will try to sell you the 6th edition (blue cover) instead of the 7th edition (white cover)
The best place to buy modern reloading is somewhere like Midway or Wideners, It's alot cheaper there.
Good Luck!
Justin