The wizard Neloth can be found at his impressive property - Tel Mithryn. The building (or rather a castle built of mushrooms) is located at the south-east edge of the island. It's rather hard to miss, being a large stronghold reaching to the sky. On the main courtyard you will most likely come across his maid and apprentice (his name is Talvas Fathryon and he's connected with the From the Ashes mission). In the local "apothecary" you can meet Elynea Mothren - a skilled mushroom specialist.
Neloth lives in the biggest of the mushrooms - the one in the middle of the courtyard. Head inside and let the magic stream take you - that way you will reach the upper level of the building. Search for the old man there and speak to him.
He will tell you that the secret of Miraak can be learned only by finding and examining the Black Books. Curiously, Neloth is in possession of one of them in his warehouse, however he won't let you see. You need to follow him to the Dwemer castle of Nchardak of obtain one of the volumes.