I read somewhere that too much fertilizer can accumulate in leafy
vegetables to the point of being toxic for human consumption, does anyone
know where I can find out more about this, especially as it relates to
hydroponics? I read this in a book on green houses, it said something
about problems with nitrogen not being used up by the leafy vegetables in
certain times of the year and thus accumulating in the edible parts of the
vegetable.

Nitrogen is poisonous? In the quantities a plant can pick it up? Hmmmm [..]

Most likely he was talking about nitrogen in the form fertilizers supply
it, i.e., nitrates/ammonium.

-Tony