Any NPC who sells Essentials books for any other level, such as the City tradeskill area or the Guild Hall vendor.
To get your rare recipe books as a Sage you can do one of the following: a) Good ole' fashion broker search. Going this route will probably put a sizeable dent in your wallet. b) Finish up the Paineel Craftskeeper's questline and then make your way over to Quel'ule in Stonebrunt Highlands. Finishing that questline will net you 7k in faction and doing the first daily quest will get you another 2.5k. Since you need 10k to buy the books, you will have to wait one day in order to purchase them from the merchant for 15g and some silver. This approach, while a little longer, is still optimal to buying them on the broker (unless of course you can find an advanced book on the broker for less than 15g). Hope that helps.
