Imagine a race of humanoid alien living somewhere in the universe evolves weird sensory organ which can only picks up frequency of light ranging from 800MHz up to 2.5GHz which is in the microwave spectrum. They can hear and talk but how would they read and write? The more primitive the tech the better.
Metals, such as aluminum, reflects microwaves. Therefore, they'd just use metals to write. It might start with something as crude as banging a metal rocks against cave walls to leave a pattern, but eventually they'd develop metal-based inks to write with against none microwave reflective items.
When you mentioned reading it brought me to think about how (regular) humans can read nature and know when it's going to snow, or if there is danger nearby.
They must be sensing a pattern which is communicating information. A natural material that can "record" electromagnetic pulses and then emit these pulses would suffice for transmitting information. You would basically "read" the material by being in the vicinity that it is pulsing. You can write to the material by modulating the pulses the material emits.