I know that the most important Holy book in Islam is the Noble Qur'an. However, I am curious if there are any other Holy books that are vital/important to Islam.
The second source of Islamic knowledge is hadith such as those compiled by Bukhari, Muslim, Dawood, and Tirmidhi. I recommend the summarized Sahih Bukhari, 40 Hadith Qudsi or Riyadh us Saleheen.