I am German, (and moreover 40+). As we Germans take everything very serious, here is my philological review of this important book:
Being a German (did I mention that already?) I can recommend this book without concerns to Germans to learn "Dirty (American at least) English" - seriously !
The other way round it is more tricky. You will learn much about German language without doubt, but many of the used phrases are not 100% correct or not really common or are out of a certain joke and what is meant, could be understood only, if the person knows that joke or you tell him/here the joke before..
Some examples:
"Schnitzelkind" cannot be commonly understood, you need to know the mentioned joke/explanation before.
Of course many words and phrases fit well, and are commonly known, for example "Grufti", "dicke Freunde", "quatschen", "Das ist g#il."
"K#ckvogel" can be good understood as an insult but is not a common word, it's more a word creation.
"Eiweisstorsten" for a man having muscles, is difficult to understand and not at all a common word.
"Tussistempel" for a common tattoo which girls are used to have at the back at a certain place, is a wrong word nearly nobody would understand- "#rschgeweih" is the correct one for that.
"Hol mir mal ne Flasche Bier" is not a question, but an order.
"srz" as a German phrase used in a chat as a replacement for the English "sry" (sorry), was new to me. It may be a good joke in the hacker community knowing details about German keyboards, but it is surely not commonly understood. I laughed after understanding it (after 12 minutes and 8 s or so), but others may not get it at all... ;-)
And so on...
The author is far from native tongue (as my English here, srz for that).
At the best pages, there is even cultural critizism in place ("We have been having ##x for weeks now, don't you think we should use 'Du' ").
So, you learn much about German, but if you want to use one of the phrases, I recommend asking a native German speaker before. Or at least google the word.
But, maybe, if you want to get in contact with a German girl, it is more "charming" ('Ist der süüüß!') to talk slightly broken German ? Dunno. Then this book is perfect. But don't ask her, if her t#ts were a birthday present. That advice from my side... ;-)