The full title of the book is „I dieci libri dell’architettura di M. Vitruvio tradutti et commentati da monsignor Barbaro eletto patriarca d’aquileggia“, so first of all this book seems to be part of a series of books. Ten books, to be exact. Also, the title suggests that the books are about architecture, but it doesn’t tell us about a specific topic. Probably it is about architecture in general. „Tradutti et commentati“ means something like „translated and commented“ which shows that the text isn’t originally from Daniele Barbaro. Daniele Barbaro just made a new version of an already existing text. I think it is an Italian version, but I’m not 100% sure.
The illustrations in the book are sometimes quite easy to understand and sometimes they are very specific and complicated which makes them impossible to understand without understanding the text. The illustrations also range from simple graphics to incredibly detailed and beautiful paintings that are similar to paintings that you find in antique churches and palaces. All these different illustrations, from scientifique to art like, are a second sign hat the book is about architecture, because architecture is a huge discipline which combines uncountable different aspects as e.g. everything from science to art.
The book is very large and the text is very small. This shows that the book contains a huge amount of information. Also we know that there are ten of these books. So probably these books cover every single little detail that was known about architecture at that time.