Useful advice on getting there, thank you!
In sort-of answer to the original question, I know it's not a book you can take with you but there's a series of lectures from 2009 on Roman Architecture on the Yale website. http://oyc.yale.edu/history-art/hsar-252#sessions
I've seen the first few and they're great.
They're also showing them as part of a free online course at Coursera - https://www.coursera.org/course/romanarchitecture