J.J market is certainly an experience. There seems to be some organization - clusters of stalls with accessories, lighting, fine art, textiles, leather goods all peppered with food vendors selling everything you can imagine. I am certainly no expert on the layout, so can only suggest setting aside a large chunk of a day to explore and be overwhelmed by the variety and quantity of goods.
What I CAN say, though, is there is a restaurant worth seeking out in particular. From the street, enter at Gate 29 or take the subway to Kamphaeng Phet, go up the escalator and do a U-turn to the right. Along the narrow path there will be tables and chairs on one side and a small cooking station on the other with more tables behind it. Stop here. Wait as long as you need to get a table.
If they think you can't speak Thai, they will hand you a picture menu (thank you!) that might still make you feel illiterate. If you do speak Thai, you get a pink slip of paper and pencil to mark your selections. The style is small plates meant to be shared. Don't expect a very attentive or hospitable waitstaff, but the food is so fresh and flavorful it's pretty difficult to care about anything else.