The number of the verb in the relative clause is governed not by the subject of the main clause but by its own subject, which. Which, in turn, derives its number from its referent, which may be either one or recipes.
If the clause answers the question which recipes?, recipes is the referent, and the verb must take the plural number: the recipes which were in her personal cookbook.
The verb only is only cast in the singular if it answers the question which one?: one which was in her personal cookbook.
More context might make it clear which of these is intended; but without that context, I think it more likely that the clause modifies recipes.
- The relativizer that and the absence of a comma demonstrate that the clause is restrictive—that is, it specifies something rather than just adding parenthetical information; so we cannot interpret this as She gave me one of the recipes, which happened to be one from her personal cookbook.
- But unlike a non-restrictive clause, which admits some intervening material, a restrictive clause is ordinarily set immediately after its referent. That points to recipes.
- Moreover, the presence of the phrase of the recipes implies that this is the important specifier for one, leaving the relative clause as a specifier of recipes. If the author had meant the relative clause to modify one it is more likely she would have written *She gave me a recipe that was from her personal cookbook.
We cannot be certain; but I think the balance of probability comes down on the side of were:
She gave me one of the [few, special] recipes which were in her personal cookbook.
This is one of the [few, special] cars that run on hydrogen.