How do gliders take off?

Gliders can be launched using a winch and a cord, towed by another plane using a tow rope, or propelled by a bungee cord.

When did soaring become popular and why?

Gliders or sailplanes have been in existence since before the turn of the 20th Century. As mentioned in the site, the Wright Brothers used gliders at Kitty Hawk, but others used unpowered flight as an opportunity to learn about navigation, understand the power and subtleties of wind and the impact of heat on air waves. Soaring became popular in Europe and specifically in Germany after World War I when a ban on single engine planes was instituted as part of the Treaty of Versailles.

Are there any books on soaring?

There are numerous books on soaring.
One of the best is from Steve Longland, who provided the graphics on types of lift for this website wrote Gliding: From Passenger to Pilot.

Another interesting book that covers early 20th century aviation history in Germany as well as discussion of gliders is A Nation of Fliers: German Aviation and the Popular Imagination by Peter Fritzsche.

If you are interested in reading more about pilots and early aviation, one of the most moving books is West With the Night by Beryl Markham. This book is the source of the quote on the site homepage.