When you say: "It seems that this one really works, and it is not possible to go around it." - on what are you basing that assumption? Their sales blurb?
If I had to guess, I would say that the only reason it hasn't been broken yet (if it hasn't been broken yet) is because it's not big enough for anyone to have put in the effort.
If the ebook can truly be downloaded and read independent of a connection to the 'net (for example I read of books available that are supposed to be "tied" to the USB drive), then you can be fairly certain that the person able to read that book could also extract the book - if they put in the effort and have the knowledge. All the pieces are there, it's just a matter of intercepting them and feeding to your own output device. Yes it may be difficult, the first time, but then the solution can be put up on the 'net for all those "legitimate" readers that would like to read their book on devices not supported by the supplied software.
If the ebook remains on the remote host and is only fed to the reader, encrypted, a page at a time, it must still - at some point - be displayed on the screen for the user to read. In that case I simply use screen capture software and - if necessary OCR - to produce my own copy to take anywhere (pretty much the same as happens with print books).
My guess is that the current solution is marketed as "secure" only because of the secrets hidden in the software, not as a result of real and tested security protocols. But that's only a guess (based on some knowledge, if not detailed expertise, on this topic). The real point is that I am quite confident that the solution will put-off your genuine purchasers without actually preventing unauthorised copies to be made by people that are willing to put in the effort.
If you want an equivalently secure "DRM" solution, go back to basics. Provide paper books only via print-on-demand. It's effectively the same thing as what the is described above (the content - words - are tied to the device - book), but at this point in time people are still familiar paper books and you are less likely to upset them.
I am not personally against DRM (to the extent that it limits a person's use as would purchase of a physical paper book), but I think it is very likely that existing ebook DRM solutions will cause more harm to the audience you actually want, than it will provide benefit to you against pirates - even in a specialist market.