Well, at least that book, scary as it seems, made me decide to come out here at last.
I'm an atheist too, very firmly so - and a card-carrying humanist. I do take humanism and respect for human rights very seriously.
I left the church as soon as I came of age, - remember trying to explain the big bang theory to my (horrified) classmates while still in children's school.
I'd found that in a book and even then it - and the theory of evolution - made more sense to me than what we were being taught in (the very religious) school we were attending.
Didn't see the need to join any organization to prove my personal viewpoints, but eventually joined the humanist organization in my country a few years back because I wanted to contribute to their strength in numbers. A substantial membership base is needed if there is to be any chance of the humanist voice being heard and not disregarded in debates on religion in schools, a possible division between state and church, etc. I live in one of the most secular countries on the planet, where religion according to polls plays next to no role in the life of about 50% of the population (though many of those do in fact passively remain on the Church's membership lists). But we've had a small Conservative Christian backlash here recently too - inspired by the US, unfortunately, but not anyway near that kind of magnitude or impact. And anyway the sheer popular complacency about the status quo makes it difficult to change things in a direction that is equally respectful of everyone's outlooks on life, or that distinguishes better between private worship and public equal respect for all. (By and large, Lutheran Christianity is still the official way to go.)