At long last, and just in time for the summer prep rush, the Math Guide is back to showing “usually ships in 1-2 days” at Amazon. (UPDATE 5/30/2020: now it’s all the way back in stock!)
I still don’t have stock to ship myself, so if you want a physical book you have to go through Amazon.
Also, we’re about 3 months until the August SAT, so if that’s your test, now is an optimal time to sign up for a 3-month subscription to the Math Guide Online. Pro tip: sign up for the free 3-day trial, and if you like it (I think you will), you can carry all your progress over into your paid subscription.