The Secret Frequency has long been my favourite part of the Darker Days Radio podcast, so a supplement dedicated to these plot hooks is very welcome.
The book provides some fascinating little hooks and elements drawing upon real-world esoterica and folklore. Each section does a good job in introducing the initial lore before going on to explore how to bring them into your games. Some of them are definitely stronger than others, with the chapter on Black Dogs in particular being one i found interesting with lots of potential (though I will admit to a long time fascination with black dog lore in any case).
All in all, a good place to start when looking for something new to introduce to your chronicle.