Follow that link for the legal language, but essentially this means that you are free to copy, modify and distribute the book as you like, even for profit, as long as you license any new variations you create under this same license, so that others are free to do the same.
If you have any questions about using all of — or portions of — this book, please contact me at cii@igc.org.
The photograph on the cover of the book — and on this website — is an edited Hubble Space Telescope image of the M101 Galaxy (also known as the Pinwheel Galaxy), which is twice the size of our own Milky Way, and about 24 million light years away. Credit for the original image goes to: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI.