The first astronauts - the Mercury Seven -- went up alone in a 9-foot x 6-foot-diameter capsule. Project Mercury got the United States into space, but once JFK announced the goal to get to the moon, the remaining Mercury flights were cancelled and Project Gemini was born.
Gemini -- the astronauts all pronounced it "jeh-MEE-nee" -- was so named because two astronauts would go up (gemini is Latin for twins). The capsule was a little larger (11 feet x 7.5 feet diameter) and could support the crew for two weeks or more. The astronauts could maneuver it more, dock with another spacecraft, and even take a walk outside. These were all necessary steps to prepare for the Apollo missions.
Gemini was the first spacecraft with an onboard computer, the Gemini Guidance Computer. Made by IBM, it weighed 59 pounds and had a 7000-calculation/second processor with (as I figure it) 159,744 bits of memory. That equals about 20KB.
I'm not sure where I picked up this book, or where you would have picked it up in 1966. It's a fairly straightforward description of the Gemini program, with sections on the spacecraft, the space suits, the astronauts, the launch vehicles (for some reason, the word "rocket" is never used), the missions (the book seems to have come out after Gemini 8, Neil Armstrong's first mission), and a page at the end on the upcoming Apollo program.
Graphically, it's also fairly conservative. Black-and-white photos, photo-illustrations, paintings, simple graphics, and some excellent line illustrations, maybe in pencil. Only the back cover has color, which makes the striking cover even more unusual.
Among the amusing entries is an illustration (see left), probably from a photo, of eleven men in Arab garb posing in a group shot. A pair of large crossed knives is visible in the foreground. On the next page, four rough-looking guys are watching a guy with a machete and a piece of bamboo during Tropical Survival School -- I'm sure this had some connection with being in space, but there was hardly room to even swing a machete inside the capsule.
I scanned the whole book, but some sample pages are below.