Why was salt so valuable? I don't understand it since it's very abundant in most areas and can even be taken from seawater. I have read several books and articles and none are very clear on this.
Salt was mainly used for preserving food, especially meat and fish. At times it was the only real way of preserving this food, so there was a huge demand for it. And while there is salt in sea water it has to be boiled out and that needs energy, e.g. from wood. And there might not have been enough wood near every coast, so not every place was suited for salt evaporation (in warmer areas that is completely different though, as you surely know). And yes it can also be mined, however that opens up risks and is not extremely fast either. Essentially it was quite hard to satisfy the demand for salt making it expensive. The ratio of demand and supply is what made the price.
"Abundance" is only one factor in cost. The other factor is the effort needed to extract the salt. Even today, extracting salt from seawater is a "nontrivial" task. Other sources such as salt mines, are only slightly less costly. So while "supply" was high, "effective" supply was low, after extraction.
Salt was historically in heavy demand. Until "refrigeration,"the only way to preserve meat for more than a few days was through salt.
So the combination of high demand and low (effective) supply made salt expensive.