coding-system-for-read is a variable defined in `C source code'.
Its value is

Specify the coding system for read operations.
It is useful to bind this variable with `let', but do not set it globally.
If the value is a coding system, it is used for decoding on read operation.
If not, an appropriate element is used from one of the coding system alists.
There are three such tables: `file-coding-system-alist',
`process-coding-system-alist', and `network-coding-system-alist'.