On any group excursion it is worth getting a set of ten-journey tickets to be shared by five people. Plano de los Transportes de Madrid which combines plans and timetables for Metro lines, buses and night buses all on one big map. This is slightly cheaper than phone boxes and a lot cheaper than phoning from hotels or commercial locutorios. Please note that the international cheap rate only starts at 10 pm, with a cheap weekend rate from Friday night to Monday morning, and the locutorios are normally open until midnight. I recommend posting letters at main post offices, as this can cut delivery times quite dramatically.
Postcards take longer than letters.
A letter to a destination within the European Union currently costs 60 Pts. In both Madrid and Barcelona you will be able to get free maps from main Metro stations and the Tourist Office. Bookshops and quioscos also sell handy, small A-Z type street atlases that give you much more detailed information, but they tend to be quite expensive. These atlases are regularly updated and sometimes you can get last year's edition at a fraction of the price in discount bookshops. Before you go, check out the Time Out web site for listings and general information on Madrid and links to other sites in Spain.