i can't stress how important it is to read both.
chenny is right, harrington is more practical, but you can't get to a decent standard without knowing the theory and psychology behind the game.
harrington 1 may be outdated, but by the time u have read that, and sklansky, you will be in a knowledgable enough frame of mind to make your own moves regardless of if you are playing turbo online or with the cowboys in the 40s!
its also important to read HOH 2&3, tho by that time you will have adopted your own style of play and will be able to filter the information out that you dont need.
It all depends on what level you are already at, if you are already competent, read sklansky first, then u can skim read HOH pretty quickly.
if you are pretty new, keep an open mind and read HOH first.
Good Luck!