In Singular Genitiv for the neutral noun "Das Buch" they wrote in Duden "des Buches, Buchs", is there a rule to choose if we must add "-s" or "-es" to the noun, or something else?!!
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The answer, as often with languages, is: It depends. The question itself however needs to be tackled in two steps:
At first you need to make sure which genitive singular suffixes are actually allowed for a given word. There are three types of nouns in the German language when it comes to this:
The inflection class of a word is also an indicator, whether only one option is allowed.
canoonet gives a more detailled overview with many more examples.
For a word where both endings are allowed -es is indeed considered more poetic as stated by accolade, but could also be interpreted as outdated. The Duden says that -es is preferred in constellations where the genetive is prefixed (e.g. des Tages Hitze) or within compounds with interfix-s (des Geschäftsfreundes).