There are two factors here: 1. illegible to whom? 2. what do you expect a book to look like? The Textura letter forms would have been more familiar to readers

There are two factors here: 1. illegible to whom? 2. what do you expect a book to look like? The Textura letter forms would have been more familiar to readers at the time, and were used widely in German until surprisingly recently. Even today we use them in English-speaking countries for short pieces of text that need to look authoritative. See the header of the manuscript of the US Constitution or the masthead of most old newspapers. More importantly, these shapes looked similar to the letterforms from expensively hand-written bibles of the time. Different scripts take different amounts of time to write. A script like this written by hand takes an enormous amount of time because every stroke doubles back on itself. So only quite expensive books would be hand-written in this way. Of course, when you're printing, it makes little difference, so might as well print with the most-expensive-looking letterforms. Here are some alternate scripts used in English writing around the same time: http://isites.harvard.edu/fs/docs/icb.topic453618.files/Central/editions/scripts.html http://isites.harvard.edu/fs/docs/icb.topic453618.files/Cent... (Source: wife studied paleography at Oxford, we have lots of fun books about this. There are far, far worse hands to decipher than textura, some scribes were truly terrible. You can also date manuscripts based on small characteristics of the hand-writing. Although, it can be a bit inexact. My wife once corrected a museum placard dating a text based on the writing. The docent argued with her about the characteristics of the script before she pointed out that the text mentioned a Saint born a century after the supposed date.)