Hi there. Well, to me, it sounds like two little girls being curious about their bodies. Many, many people have experiences of exploring their bodies like this during childhood. Sometimes alone, sometimes with others of the same or opposite sex.
It sounds like you have a tremendous amount of guilt about it. Were you taught that touching your own - or somebody else's - body was wrong? or dirty? Is the guilt because it was with another girl?

Young children of that age are fascinated with their bodies, and naturally explore them. (Imagine if they didn't, and their 'nether regions' were completely foreign to them until adulthood!) I guess in your situation, you and the neighbor girl found something that took your exploration a little further than you would have gone without that input of adult information. Were you 'bad' for asking the girl to touch you? Nah. Were you bad for *liking* it? Nah. Sexual touching feels good. Even for five year olds. You were a normal, curious five or six year old girl, doing nothing more than what countless other normal, curious five year old girls would have done under the same circumstances. (I work in a preschool - trust me. I have *seen* things!)

If you ever have a five year old girl of your own it might pay to try to keep any sex books out of her way tho, eh? Your exploration in itself was perfectly normal, but the book likely took you and the other girl into a realm that no five year old is ready for.