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Trail Life and Tarp Book, both by Ray Jardine and available at his website, are excellent introductions to backpacking, especially the lightweight version that I prefer. Readers of my website will note that I consider Jardine to be something of a guru and make reference to him constantly. I think my own designs have improved on his, at least for my purposes, but I definitely recommend everyone see for themselves what Jardine has to say. Beyond Backpacking is an earlier edition of Trail Life, and is the book that inspired me to take up backpacking in the late 1990's, after pretty much giving up on the outdoors after miserable experience with heavyweight backpacking with the Boy Scouts as a teenager in the 1970's.
The Backpacker's Handbook by Chris Townsend (2011 or later edition) is another excellent introduction to all aspects of backpacking techniques and equipment. Includes an extensive bibliography, which probably includes every book on backpacking worth reading. So no need for me to list any more books. Rather, I'll just refer readers to this bibliography of Townsend's.
Viajar A Pie for some more information about hiking in Spain.
Kayak is my primary source for airfare. Kayak can be setup to automatically search Priceline, but if this changes, then search there separately. ITA Software provides the underlying data used by Kayak and the other airfare search sites, and some say it is a better site for searching. I haven't used it much myself, however. SkyScanner is also highly recommended by many people.
Kayak omits many of the discount airlines (Southwest Airlines, Easyjet, Allegiant, etc). So do an internet search on "low cost carriers" to find all airlines that service a particular airport, or use Flights From. Or search for airport website, which usually has page listing all airlines that service that airport. For airlines not searched by Kayak, visit airline website directly.
TUI Fly is an example of a charter company. These charter companies sometimes sell their excess seats at very low cost. Note that both low cost carriers and charter companies often go out of business with little advance warning and sometimes no provision made for existing ticket-holders.
GoEuro is a good travel search engine for travel by train/bus within Europe.
Cost of Living Calculator, for various cities around the world.