An invaluable app during our travels has been iBook. Designed both for the iPhone & iPad it provides a convenient way to read books generally, such as a favourite novel when flying, and an easy-to-use reference guide when sight-seeing. I find this much easier than carrying heavier hard-copy books, especially when baggage weight allowance is an issue.
The series of “[city] Sights” by MobileReference is one example of electronic books we used whilst in Rome. Yet we also have copies for all the other major cities we will be visiting.
It supplied a helpful and relatively detailed explanation of the various sights as well as suggested itineraries with maps. Pretty good value at about A$2 each.
The main travel guide companies such as Frommers and Lonely Planet also produce electronic versions of their hard-copy guides but appear to be priced at A$25 – A$35 for each country.