An inspiring book I've been reading/skimming is "The Millennial Project" by Marshall T. Savage. It is an 8 step plan for colonizing space in the next millennia. The author is very well read and educated (the book has the largest bibliography and end note section I've every seen). It tends to be a bit idealistic, which is to be expected for the subject, but is very informative and feasible. Savage has also started the First Millennial Foundation to bring his plan to fruitation. I recommend the book to anyone interested in either space colonization or environmentally friendly engineering. All of his projects are designed to work with nature, not against it like most large projects.