Fishing is super easy.

1-250 in Stormwind/Orgrimmar.
Do the quest.
250-300 in SW/Org (300 is as high as you need for ZG fish boss.)
Buy lures.
300-375 at Cenarion Refuge in Zangarmarsh (feltail delight and blackened sporefish sell decently in the AH.) You can go here with lures around 250, but you'll still miss quite a few till you get to around 300 before using a lure.

Unlike every other profession, fishing skillups is in no way related to the difficulty of where you're fishing. You get a skillup every X successful catches, where X starts out at 1 and eventually gets to something like 8 or 9. For pure speed, your best bet is to fish only in SW/Org so you never miss a fish.

I believe the ZG book has a 10% drop, so with the 3d raid timer it may take you a while to get one. I managed to find one on my server for 1k, so my mage has it .