So far as we can tell, books, properly speaking, didn't exist until well into the first century A.D. In fact, at the turn of the era between B.C. and A.D., written materials in any form were rare. Only between 2% and 15% of the populace were literate. And normally, when writing was done, it was a purely practical or business matter of requests or accounting. There were, of course, treaty documents and written laws as well, and public inscriptions and signs, but that was about all that appeared in written form. The world Jesus and Paul lived in was an oral culture that placed a premium on the spoken word.
There was, however, an increasing need for scribes, or, as we would call them, secretaries, especially in cultures with sacred texts--such as Jewish culture. The wonderful finds of the Dead Sea Scrolls at Qumran show us that there was something of a developing "book" culture in the Jewish world. In the Greco-Roman world, secretaries helped run the empire, and by the time of Cicero, published letters were sent out for public consumption. Thus, it is fair to say that Christianity arose at a time when what we would call publishing--making multiple copies of a document for public consumption--was beginning to transpire.
The form that most documents took in antiquity was words written on papyrus scrolls, usually by a scribe. In Jesus' day, the sacred writings we know as the Old Testament would have taken the form of scrolls. And so would the earliest gospels.
Scrolls were not the ideal medium for transmitting the written word. Writing on or copying a scroll was a laborious process that took quill pens, tar gum, water, and powder to keep the ink from running, to mention only some items. Papyrus was expensive. The maximum number of words one could get on a scroll is about what we find in the gospel of Luke.
Scrolls were prohibitively expensive for most people to own, and they were also hard to read. In order to make the most of the expensive papyrus, a scribe would usually not separate words, sentences, or paragraphs but just run them all together. This, of course, led to problems in reading comprehension. For example, what does the following sentence say: "JESUSISNOWHERE"? Does it read "Jesus is now here," or does it read "Jesus is nowhere," or was it written by a nihilist with bad grammar and read "Jes, us is nowhere"? This is but one example of the difficulties in deciphering the early Greek manuscripts of the New Testament.
It was hard to find one's place in a scroll--there were no pages. This led to the practice of tabs being attached to mark the beginning and end of a document. Probably the headings to the gospels ("The Gospel According to Mark," etc.) were originally merely tabs attached to the scroll. This means the original text of the Synoptic Gospels (Matthew, Mark, and Luke) are formally anonymous documents.
The manuscript book, or codex, which came to the fore sometime in the first century A.D., was a significant improvement. It was actually possible to find things rather easily in a manuscript book if one made distinguishing marks on it. Of course, the Bible was not divided up into chapters and verses until the early Middle Ages, but in the first century, marginal markings might already have allowed readers to go back and find important and favorite passages much more easily. Books were also much less flimsy and cumbersome to deal with than scrolls.
Apparently early Christians took advantage of this fact. We know for certain that there were collections of the four Gospels together in book form in the second century, for Irenaeus and other sources mention them. But this was only a collecting of gospels that had already been in book form as individual works for some time before then. Some have even suggested that the gospel of Mark was the first real book to ever be made. And 2 Peter 3:15-16 likely refers to an existing collection of Paul's letters already extant in the first century. Such a collection, due to length, would have had to have been put together in book form.
For Christianity, an evangelistic religion, the rise of the book trade came at just the right juncture to help spread the faith. Thus, if one asks how it is that Christianity spread in both its earliest days and since then, one appropriate answer would be--by the book.