What is the ruling on doubts about the existence of Allah and the Muhammadian Message that come to mind involuntarily, and are there specific books or lectures that help in such cases?

You must be diligent in praying in the mosque because it is one of the greatest factors in increasing faith, and it is an individual obligation for men unless there is an excuse. You should also resume memorizing the Holy Quran, for the Prophet, peace and blessings be upon him, said: "The best among you is he who learns the Quran and teaches it." You can seek help from a trustworthy shaykh to assist you with memorization. These whisperings will not harm you as long as you hate them in your heart, and your hatred for them is a sign of clear faith, as stated in the hadith of Abu Hurayrah. The cure for whisperings is to ignore them, and if they come to you, say: "I believe in Allah," and then stop. You can also seek help from easy-to-understand books on creed suitable for your level of knowledge to strengthen your faith, such as the book "Aqeedat Al-Mu'min" by Al-Jazairi, and the "Aqeedah" series by Al-Ashqar, as well as the writings and tapes of العلامة Al-Uthaymeen.