Where can I find my license key?

Two places:

• Right after purchase, on the confirmation.
• Any time, under My Books → Purchases & license keys. This list shows every key tied to this browser and to your account, grouped by book.

If you sign in, your keys sync to your account, so they'll be in that list on any device you sign in on — the safest way not to lose them. A Complete Book key looks like BBK-XXXXXXXXXXXX, and an ARC Delivery key like BAS-XXXXXXXXXXXX.