What is a good sefer or even book that helps people with a problem with getting angry?
The range of sources that discuss anger in Jewish tradition is huge.
There is a sefer I saw called Haser Ka'as Milibecha, (Translated as "Remove Anger from Your Heart"), a classic mussar work compiled by Rav Avraham Tubolsky, shlit"a.
Gan Emunah (available in English as "The Garden of Peace") discusses it, and I bet Gan HaShalom does as well. Bilvavi Mishkan Evneh (Available in English as "Building a sanctuary in your heart") also discuss this.
Oroḥt Zadikim discusses it (but to take full advantage of that sefer, you have to have an understanding of how to learn mussar), and Alei Shur provides exercises regarding anger (and lots of guidance about the mussar derech in general, and lots of exercises for improving other middot as well -- this can help with using Oroḥt Zadikim).
The hard part isn't finding books or reading about it -- it's internalizing what the books have to say.
There's a book by Rabbi Zelig Pliskin called 'Anger: the inner teacher', it's very clear and set out step by step.
The Dance of Anger. The subtitle shouldn't discourage men, either.
Igeres Haramban read and studied once a week has helped me for many years.
The Trail To Tranquility by Rabbi Brody is my favorite: here it is on amazon