English translation of Arthamulla Indhu Matham (Meaningful Hinduism) - a Tamil book by Kaviarasu Kannadasan.
The simplest Shiva Mantra anyone can recite anytime is:
Om Shivaya Namah: (or) Om Nama Sivayah:
ஓம் சிவாய நம: (or) ஓம் நமசிவாய: