I know someone who is teaching himself German from the original copy of Assimil printed in 1957. He thought to himself, "wouldn't it be hilarious if I spent years learning out-dated German?".
Is this book or any other older book good for learning or is it far beyond of how Germans speak today?
There have been multiple changes in spelling rules between 1990 and about 2005. This included raw spelling (i.e. Schifffahrt instead of Schiffahrt) but also changes in capitalization and setting commas. I would strongly suggest to use a book later than 2005 for learning current german, although for colloquially talking, the differences are not that big.
Most aspects of the language haven't changed since 1957, but some details have. If these details are crucial to you, you should use a newer textbook, if not, feel free to use the one from 1957:
-erweil, … have been accepted in casual language.It depends on the context you are preparing for. If you need it for professional business, and there will be a lot of written communication, go with the newest books. Same goes for academic, because of the recent changes to spelling, etc. However, if you are going on vacation and need to know how to order food or arrange transportation, you will manage just fine with any book you can get your hands on; in these cases, it doesn't matter if you use the ß or ss in a word (as it will likely not be written anyway).
Of course they are still useful. Languages generally change slowly. The more formal the register, the slower the change. The latest slang may have changed, but such books generally don't attempt to cover slang anyway. Even 19th century texts are not that different, just as in English.
