The Master of Shadow makes your familiar into a incorporeal shadow construct that gains abilities like a familiar.
Uh... no, it's an elemental and gains abilities like an animal companion (bonus HD and stuff). But whatever, that's not the real issue with Master of Shadow: It requires a feat to enter (Shadow Familiar, which isn't that great by itself), which requires three levels of Shadowcaster. Sure Master of Shadow advances spellcasting, too, but there just isn't a sensible way to enter.
Now a sensible DM might allow you to enter with a normal familiar that wears a Collar of Umbral Metamorphosis, because Shadow Familiar only gives a Dark templated Familiar. Which... is just expensive, but not nearly as bad.
For real shadows, take Master of Shrouds and any early entry trick you can get by your DM, and follow the handbook.