I bring seforim along. Last year I read Rav Saadya Geon’s Emunos Vdeos on Tshuvah – it sounds like it would be heavy reading but No, it was fascinating. Its not much – he is very sparing of words, but thats the beauty – you read a thought, then turn it over in your mind as the Chazzan sings and everyone is happy.
Some other candidates are the Rambam on Tshuva, the Beis Hashem of the Mabit on tshuva and so on.