Category of the First Term in the Partitive Construction

Are the words in bold type in the following sentences determiners?

1. One of the books was written by X
2. I want two of those
3. 8 percent of the population has X
4. I ate some of that cake

In a treatise on partitives, the author identifies them as determiners. But I don’t see a noun following these ‘determiners’. Without a following noun, they more resemble a pronoun referring to that absent noun.

Online Oxford Dictionary defines partitive as:

1. A partitive construction
   1.1. A noun or pronoun used as the first term in a partitive construction

From the definition it seems that the first term, which I think is the word or a group of words before of, is either a noun or a pronoun.