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I found the Stephen Anderson method book(s) to be very useful when I was starting on the alto. Some people on this site will disagree with his approach - he writes out short etudes in bass clef in E flat, with the same exercises on the opposite page in alto clef (in C). I liked them a lot for two reasons:
1. They were short (2 or 3 lines) to start, and were fairly intuitive to start. They do get a bit longer and more complicated as you go through the book.
2. Starting out by treating the instrument as a transposing horn meant I could play tunes immediately. I could concentrate on tone, articulation, and especially TUNING to find a certain comfort level with the horn. Then, I would look across the page to the same etude in concert pitch alto clef to train my reading skills on the instrument.
I think one tough thing about starting the alto is getting past the frustration of feeling like a novice again after having achieved some skills on tenor. With this method, you're playing some simple tunes right away, so you can just concentrate on cleaning up your instrumental skills for the horn in a melodic way - more enjoyable that the (also) very important time that you need to dedicate to scales, etc.
Anderson is (was?) a professor in the midwest (Kansas?). His books (I believe there are 2) are still on Hickey's site.
I would also give a thumbs up to using drones while practicing scales and arpeggios.