Well, chances are that if the book contains void main(), it was probably written over 6 years ago. So it will probably include lots of other non-standard code. The problem is that it's not 1996 anymore. The language has changed quite a bit in 5 years. I find it annoying that teachers and professors, for whatever reason, continue to use pre-standard books to teach the language. If the book was written after '98 and still contains dinosaurs like void main and headers ending in .h, then the author needs to wake up and smell the 21st century. It is my opinion that we as programmers have a responsibility to keep up with the language standards if we want to get the most out of it. Standard C++ is a much better language than pre-1998 C++.
Either way, it doesn't discount the book right off as being bad, but I'd be wary of it.