1. English recipes: English recipes may be thin, because English dishes may be very simple, and many dishes may have only one or two recipes.

2. American history: American history may be very thin, because American history may be short, and many historical events may only be described in a few lines.

3. German jokes: German jokes may be thin, because Germans may be humorous, and many jokes may have only one sentence or a picture.

4. Italian War: The Italian War may be very thin, because Italy may not have participated in many wars, and many wars may only be described in a few lines.

To sum up, according to the information provided by the title, these four books may be very thin. So, we are not sure which one is the thinnest.