Do i need any invitation letter from my visitor if I am traveling to USA as a tourist? Also, this visa was issued to me by my company, can I use the same visa to travel again to USA for tourism purposes? If yes, what additional documents are required from my end?
If your visa actually says "B1/B2" under "visa type/class", then it's a combined business and tourist visa, which is valid for either purpose. It can be used again if it is still valid and was issued for multiple entries.
If it says only "B1" (which appears to be rare but possible), then it's a pure business visa and not valid for tourism.
You don't need an invitation letter for ordinary tourism (after the visa has been issued), but you need to be able to answer questions about your plans and itinerary convincingly at the border. Having tickets and hotel reservations ready to show is strongly recommended. If you're staying privately, have full contact details for your hosts with you. (Having a letter from your host could be helpful if you're depending on them for financial support during your visit and can't show sufficient funds of your own at the border).