Legal beagle is quite commonly used here. More so I think than 'legal eagle'.
Quite agree that pedantic authors, or ones whose text is too liberally littered with obscure or unintelligible words, are a bit tiresome. I like an author who draws me in to the world they describe. But on the other hand, you do learn a lot and expand your vocab, if that's what you want.
Having said all that, I confess that at the moment, I'm reading Enid Blyton's St Clare's school stories!!! Very 'jolly hockey sticks', a fascinating insight into boarding school life in the thirties. I'm going senile I expect.