"Navnatha Charithra: Nithya Parayana Grantham" in Telugu written by Sri Isukapalli Sanjeeva Sarma is available from Mohan Publications, Opp Ajanta Hotel, Kotagummam, Rajamundry-1, phone 2462565. Please call them before placing an order, as some times the book goes out of stock. I did not come across any book in English. There are books in Hindi, Gujarathi and Marathi. Books like "Navnath Bhakthi saar" are little difficult to comprehend, Better to begin with simple books and later graduate to Bhakthi saar.
I have seen temples in Mangalore, which are closely connected to Nath cult- Kadri temple (Mangalore city) and Adi Parasakthi temple (where a huge statue of Guru Matsyendranath on a small hillock overlooks Adiparasakthi temple; and Guru Gorakshanath idol is in Guru sthan within the temple). Mangalore is named after Queen Mangala Devi, who as per the legend was married to Guru Matsyendranath.
Jaya Guru Datta