product recommendation - Sefarim to learn in Elul

Which sefarim are traditionally learned in Elul*, because they have a special significance to the mood and message of the month?

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*_{Or even without a tradition, which sefarim are good to learn during this time?}

Answer

I forget who told me this, but I heard that Tomer Devorah is traditionally studied in the month of Elul.

I've been told that the Bobover Rebbe told his chassidim to learn Tomer Devorah this year, and that Rav Moshe Wolfson told his Kehilla the same a few years back.

This practice makes sense, because the first chapter of Tomer Devorah is about Hashem's 13 Middos, and how they should be taken into practice, which is a focus of the month of Elul and Yamim Noraim.

(Hebrewbooks, print Hebrew, print English)
_{These are just examples, but you can find more of all the above types at Hebrewbooks and other seforim websites; I linked to them for illustrative purposes only, not necessarily because they are the best of each type.}