Books before the printing press either had to be hand-copied, or they could use an older version of the printing press called "moveable type". It's basically a system of stamps with a stamp for every letter of the alphabet (and all the periods, commas, and apostrophes we'd need as well). They would have to be hand-stamped into the book. This process (whether by hand or moveable type) was very slow and labour-intensive, therefore books weren't exactly cheap.

In terms of readership, his book was published at the beginning of the Renaissance, when the upper merchant class was not only emerging, but becoming more and more interest in education and being seen as someone learned. Young men of the upper and business classes were expected to be polymaths, having skills and interests in multiple languages, arts, music, and sciences. This desire for knowledge made a book about the east extremely interesting. Remember, before this, people didn't really know a whole lot about what was out there so hearing about a strange culture with barbaric traditions and one man's amazing journey through it written in a language they could speak made it extremely intresting and popular. It also was a bit of a status symbol that you were able to buy a book, and talk about a particular popular book with your rich pals.

And no, books like this were written a lot during this time, but not any about this topic or to this mass appeal in general. The Travels of Marco Polo are incredibly unique in popularity at the time (other than the Bible)