Starship Troopers is a great fiction book about the basic contention of why men fight. Heinlein said it wasn't meant to answer the question definitely, but the book is thought-provoking and serves to ask the reader to at least question the relationship between war and politics. A main criticism of Heinlein is that he is a hardline fascist and injects his views in his writing, but I thought the book was written without this major bias - the reigining political body is a meritocratic republic for crying out loud! Aside from that, the book is half-adventure, half-philosophy and gives high schoolers a good base to start pondering about practical politics vs. idealism.
The only drawback to this book is the assumption that the government works without the war that is raging throughout the book. It is implied since the book starts before the war, but all philosophy-related questions deal directly with war and neglect the question of what military soldiers do during peacetime. They are portrayed as being political busybodies but unlike the portions dealing with how the government works in wartime, peacetime details are not satisfactorily fleshed out.