1-There are eight dozen such books here.
2-There are eight dozens such books here.
3-I only need two dozen these books on this shelf.
4-I only need two dozens these books on this shelf.
I only need two dozen of these books. (corrected) Since the words two and eight imply that there are more than one dozen the plural form "dozens" is not needed. An example using the word dozens would be, "There are dozens of clouds in the sky". The sentence does not state how many dozen there are so the plural form is used in this example.