Hi Anastasia,
Unlike PMP, the PMI-ACP does not have a single key reference for the exam. Instead, there are about 12 textbooks and references that are recommended by PMI and you can find the list in PMI website. It is probably not very common that PMI-ACP candidate read all of them. In my case, I relied more on Mike Griffith book and prepcast online course ad mock up exams, assuming you have a previous experience working in agile-based projects.
I also recommend you walking through lessons learned forum and I am quite sure you will find a lot of very helpful guidance. Best luck !