Hi Maxim, здрасти!
I have the "German in 30 days". I bought it a long time ago, longer than 30 days! And I speak practically no German :)
I found it could be a good reference, especially for people who learn well from books. It covers the basics of grammar, has exercises and vocabulary. But nothing in great depth, it's doesn't try to cover every possible rule and exception.
For myself, it didn't quite work because I learn better by hearing and seeing, than by reading.
If you have any questions about the book itself (how it's organized or its contents), let me know!