est couvert de posters
Des = de les
It is used in the same cases in which you would use a definite article. That is, you would use "des" if you are referring to a group of _known_ posters of which you have already talked. Let's say that in the book the main character is looking for a collection of expensive posters. Can't find them and then she realizes that someone has pasted them on the wall. Then the author would write that "Le mur était couvert des posters", that is, "of those precise posters the character has been looking for".
However, if the idea is simply that the wall is covered by some unknown, unspecified posters, then you can't use the article "les" and, thus, the preposition is plain "de".
Notice that the same considerations apply to the second "de", the one in front of "groupes". If those bands had been referred to in advance, "des" would occur instead of "de".
Hi, thanks for the explanation! Isn't des also plural of the indefinite article though? What I still don't understand is why you can actually use couvert de posters without any article. There are not that many cases when you wouldn't use any article in French and the one in the questions isn't mentioned in any of my grammar books. I also searched the Internet and couldn't find any explanation.
As to groupe de.. - I always thought that we would never use DES after expressions such as groupe de, mode de, beaucoup de etc. etc. How come that is possible here?
Don't mistake "groupe de" and "de groupes", it's not the same thing. For example, the sentence could have been:
"... couvert de posters des groupes dont je t'avais parlé"
Yes, "des" can also be the plural of the definite article but it is not in the above sentence for either occurrence (for the three of them in fact, including the one after murs) of the word. A "de" preposition is clearly required in all cases and, thus, "des" must be understood as the contraction of "de" and "les", as I explained.
There are not that many cases when you wouldn't use any article in French
It's not a question of frequency. Articles are required in some context but not permissible in others and you must understand what meaning is conveyed in each case.
One way to think of this is as follows:
You often get "des" as a indefinite article as you mentioned. Such definite article can be thought of as a kind of null article: it indicates the absence of a determinate object, the absence of a definite article. It's precisely the existence of such null-valued article that makes examples of absolutely no determiner in French rare. As a comparison, in Spanish you would find no article in most cases in which indefinite "des" would be used in French.
Now, in the sentence above, if you didn't have the "de" preposition, you would need to use the other kind of "des", the indefinite, for example:
Un des murs de la chambre est couvert de posters. Ce sont _des_ posters de groupes dont...
Can you see? That would be an indefinite "des". So, if you want to paste both sentences together you would end up (in theory) with:
posters de des groupes dont *
That would be the only possible article that you could use in this case. However, this is not possible in French, it is ungrammatical, which is why I've added a star to the above sentence.
You can think of the whole thing this way (if you find it helpful)
INstead of "de des" you write a simple "de". That is "des" as an indefinite article is not possible after the "de" preposition so it just gets deleted and you end up with no article.
* "de des" becomes "de"
*"de les" becomes "des"
Thank you for the explanation! I think I get it now :)