I have Professional Blackjack (Wong), Blackbelt in Blackjack (Snyder), Blackjack Secrets (Wong), and Blackjack Attack 3 (Schlesinger).

What book would you buy next? I realize that there will be repeated material in all of them, but I like to buy others anyway because there is always a few pages or at least a few sentences that provide new information to me or maybe make me think about something differently. This, for me, makes them worth buying.
Books that talk about cover are more preferable. I have been counting successfully for a long time so I get the math and the index plays. However, I don't have a lot of casinos in my area so anything that talks about good cover is valuable. What I have been doing so far has been effective in my opinion but if there were even one additional element of cover that I could add, it would probably be worth it. Note: I am aware of the cover plays in Don's book and implement them here and there.