I asked a former colleague who's now web designing at Yahoo for AJAX book suggestions
Head Rush Ajax is fluffy -- the content to graphical-eye-candy ratio is quite low.
Over the next few years, AJAX will start to transform the web, which will in turn transform online e-commerce.
(Drag-and-drop shopping carts, anyone?)
If you develop your e-commerce site internally, start feeding AJAX books to your developers now...