Grammatically, both are correct; however, questions of usage overrides grammatical considerations.
Some free compounding is possible: the meaning is quite clear and a dictionary definition is not necessary; thus such compounds as "water level", "air temperature", "bottle content", etc. may not even appear as entries in some dictionaries but do have a certain degree of currency, and are even often more or less dominant; they vie with the prepositional phrase constructed with "of", and sometimes even have an equivalent genitive construction.
The various remarks can be corroborated in the following ngrams.
book cover, water level, air temperature, bottle content
It can be seen that often enough the compound form is the form mostly used but there are exceptions as for "bottle content" (which does not feel perfectly idiomatic, but which, after verification is found quite acceptable). This state of affairs is exacerbated for "death cause"; this form is so little used that it feels unidiomatic and that one will rather say "the cause of death".
There is as yet no rule that can help in deciding which is correct in all cases or to what degree one is acceptable or not; a feel for those notions will be acquired only through prolonged use of the English language.