I've got 5 Moroccan cookbooks (and am going to a Moroccan party this weekend!) my two favorites are:
Cooking at the Kasbah by Kitty Morse (many delicious, easy recipes and 7 specifically tagine). It's very accessible and my absolute favorite is called "Garlic Beef with Cracked Green Olives" which is ridiculously simple and so flavorful.
the Food of Morocco (a journey for food lovers) by Tess Mallos. This i got on a remainders table at B&N several years ago and was put out by bay books. it's still available on amazon, but i notice it's a different edition than mine. It also has delicious recipes, and 13 specific tangines. Of those, several are lamb. This book is lovely to read as well, with beautiful photography and a dessert section that makes me want to get on a plane immediately just to get to a Moroccan bakery!
good luck! Moroccan food is so good!
Mediterranean Clay Pot Cooking by Paula Wolfert.
It's not strictly tagines, but also Chinese sand pots, Cazuela's, La Chamba, terra cotta bakers like the Romertopf ect. It's a wonderful book and my favorite tagine recipes are based on ones in this cookbook. A nice primer on the types of clay pots, and how they should be cared for before, during and after cooking in them.
I have some other strictly tagine cookbooks but this one is my go to.