Could you tell me please where can I find a store in Rome to buy a travel guide such as Lonely Planet?
I hope you there's one near Termini station because that's the only place I know :)
There is a surprisingly large bookstore, named Borri Books in the railway station. It is on the side of the bus station on the Piazza dei Cinquecento. There are two floors, so go upstairs if you don't find the travel guides immediately on the ground floor. I think they are on the upper floor, but my memory might fail me on this. The store is open from 7am to 10pm (8am to 10pm on Sundays).
If you click on "Orari d'apertura" on the website, you'll find a map with the location. Like @mts said in his comment, you can't miss it.