The Siddhant-Bodha the book on "Adwaita" (religion and philosophy) by a Muslim saint--

          You all will be surprised to know that in Maharshtra in the district Beed, on the Beed Aurangabad road there is a small town called "Shahgadh. There in Shahgadh there is a tomb of Shah Ji Baba or Shah Muni. This saint was Muslim by religion and still believed in Adwaita philosophy, Mahanubhav Panth and also believed in worshipping Lord Krishna. Five different Muslim families who always believed in the preaching of Shah Ji Baba or Shah Muni look after his tomb. Every year on Chaitra Saptami and Ahstami (in April) 'punyatithi' or 'samadhi-din' (death anniversary) is celebrated. Punyah-waachan or the religious reading of the Siddhant-Bodha the only book written by Shah ji Baba is done. The followers and the disciples of Shah Muni very strictly don't eat non-veg and don't smoke or drink during that month. Mahanubhavi/Adwait wadi preachers are invited and lectures on Mahanubhavi philosophy, Adwaita etc. are arranged.

           This saint Shah Ji Baba was born in very religious Muslim family who believed in the equality of souls or Adwaita. His great grandfather Shah along with his wife Amina, migrated to Prayag from somewhere in the extreme north. (Exactly from which place is not traced.) Then later he got settled in Ujjain. As they both were issueless, they made a 'mannat' or asked god to gift them with a pious son at Lord Mahakal's temple in Ujjain, which was soon fulfilled.They were blessed with the most handsome son named as Janu Ji. Hence Shah and Amina both became the great devotees of Lord Mahakal who is considered as Shiva-avatar (the form of Lord Shiva). Their son Janu Ji was a very religious man. 
          Janu Ji and his wife Mandau got only one son who was named as Mann. (Later on this family migrated to 'Siddhatake' in the district 'Ahmadnagar' in Maharashtra.) 
           Mann was married to a religious lady who was called as "Ammai" or "Ammadi". Ammadi believed in worshipping lord Vishnu and she taught her only son "Shah Ji", Vishnu Bhakti i.e. to pray and worship Lord Vishnu. 
            This son later who believed and practiced both (Hindu as well as Muslim) the religions, became Sanyasi or sage in 1778. He made Munindra Swami his Guru. He traveled all over the Mahrashtra and the different states of India. He preached "Adwaita" philosophy everywhere. He was a very staunch follower of "Adwaita". He wrote a book called Siddhant-Bodha in 1795, while he was staying in a peaceful village called as "Patharimundan". This book contains 50 chapters and 9,858 religious couplets. All this literature preaches "Adwait" philosophy. He stated that "--the crux of my preaching is that there may be several arguments/fights take place in the world amongst different religions, different religious opinions, different sects, different religious philosophies but "Adwita" defeats and will always defeat every one. The one who practices this "Adwaita" philosophy solely in his life is always content, complacent and happy in the true sense. Like the cobra that slithers everywhere restlessly but eventually finds peace only when it pushes itself calmly under the ground, one's sole finds peace only when he/she gets solely buried into the "Adwaita". My book will certainly fulfill the wishes of the readers and will clear all the doubts if they read devotedly."

           His followers claim that if you read the whole book three times in your whole life you part away from sorrow, depression or any other negative emotions and be a happy person.