Considering its prefix, syntax, and context, indeterminate in this excerpt from “The Garden of Forking Paths” means overwhelming.
Indeterminate means not exactly known, established, or defined. Within the context of the excerpt we might consider that the word indeterminate means messy or overwhelming. However, since it is able to be examined and is contradicting, the best choice is overwhelming. There is no other evidence in the passage to suggest that the books were in messy heap.