There are many excellent books about the grief surrounding pet loss. This list will grow over time and I encourage to get in touch if you have come across a really good book or resource that has helped you at a time of grief for your pet.
When Children Grieve: For Adults to Help Children Deal with Death, Divorce, Pet Loss, Moving, and Other Losses by John W. James
Soul Comfort for Cat Lovers: Coping Wisdom for Heart and Soul After the Loss of a Beloved Feline by Liz Eastwood
Blessing the Bridge: What Animals Teach Us About Death and Dying
by Rita M. Reynolds
The Rainbow Bridge: A Visit to Pet Paradise by Adrian Raeside
The Loss of a Pet: A Guide to Coping with the Grieving Process When a Pet Dies
by Wallace Sife
Goodbye Friend Gary Kowalski
Lost Companions Reflections on the death of a pet Jeffrey Moussaieff Masson
The Pet Loss Companion: Healing Advice from Family Therapists Who Lead Pet Loss Groups by Ken Dolan-Del Vecchio and Nancy Saxton-Lopez