Books weights usually (but not always) have less contrast than their roman counterparts. This makes them more readable for running text.

There are some old threads with lots of details about this. Try using Google with the site:typophile.com keyword to find them—the Typophile search engine doesn’t work very well.

Here you go:
roman, book, light - when to use?
Book-Regular weight conflict
Choosing the right weight: Text vs. Regular

Book v Roman!? Why fight - just use a Book-Roman...

http://www.myfonts.com/fonts/fontfont/ff-meta/book-roman/

Thanks-- the search was more fruitful once I knew that roman and regular were often synonymous...

Using 'roman' to refer to a specific font within a family, e.g. synonymous with regular, seems really dumb to me, since as an adjective it could be legitimately applied to any weight of upright type in the roman style (as distinct from blackletter, civilité, etc.). A 'book' font in this style is just as much roman as any other weight.

I think some of the confusion in the naming of fonts within a family is because some of the terms refer to weight (e.g. light, regular, bold), whereas others I regard more as optical size descriptors (e.g. book, text). When I see a font described as book or text, I expect that it's meant to be used at around 10-12 points in body text. So you could have book (or text) light, book regular, book bold. John Hudson is right: "roman" is simply a style descriptor--i.e. bold roman, bold italic. If there is no style descriptor, it's assumed to be roman.