I know that the bonus mission for "Saving Private Smith" comes with the invasion option, if you get all the code books, but can I get all the books, quit, go onto the next mission, maybe come back to this and do it later?

I kind of want to get to The Castle, and don't feel like the arduous task of stopping those tanks right now, but I might want to in the future.

If I'm handcuffed here I suppose I'll get right to where Smith radios HQ, send Lupin to the last Bonus Book, and then make a Save Game. That way I can skip the book, go to the castle, later on come back, reload, grab the book, play the invasion.