Paulo, while I don't own any of the Osprey books, I've looked at them a number of times, and to the best of my recollection the drawings are in 1/72 scale...I could be wrong.

Perhaps someone here owns one where you can get a diffinitive answer.
BTW, while looking around for books today I came across one and thought of you. It's a book about the Brazillian Expeditionary Force in WWII!!! :-) It's titled "Na Segunda Guerra Mundial" by L.B. Montiero. Looks real cool, and best of all it comes with a decal sheet in 1/72 and 1/35th for their markings on your models!!!! Unreal! They look great! Just scroll to the bottom of the page on this link. The decals have a link too along with the publisher where you can get it.
Of course if you already know all this...err...uhhh...disregard all the above...lol.
http://www.burleehost.net/ontheway/bookreviews/bookreviews.htm