I think it may wary depending on where you are. But here it is mainly like this:
If you have bought a copy of that book - It is yours! If the media that holds that book breaks, the book is still yours and it must be your right to copy it for that purpose (backup). You can lend your book to your friend. Your purpose of buying the book was clearly to read it. So if you alter the format to something readable to you that is fine.
Now some hard and software producers pretend solidarity with the artists and want to manage their rights digitally (drm). The mere fact that drm exists on Hardware or in software does not make it illegal for you to make a copy or to alter the format of your media.
You have first and foremost bought a copy of that book (not a CD or media). And no one should deny you your obtained right to read that book.
The same thing goes for Music.
So yes if you can remove the drm to get a readable book - that you have payed for- it is legal here.
It would be illegal to sell you something that you cannot read, and calling it a book.
Obtaining and playing music that you have not payed for would probably be illegal though. It could be financed by adds etc so that is no certainty.
If you have agreed to some conditions on some sites you must check them - this might not apply then.
To sum it up: It is not illegal to break the drm. It is mostly illegal to obtain films and music and books you have not payed for (and that is what drm is supposed to control). But the converter itself is legal.
PS
Since it is not FOSS I am still undecided if I should regard that link as spam or not

I guess it will depend on your motives for asking which I am sure you will share with us now
