Question: I'm an American and looking to travel to Iran overland by way of Turkey. Wanted to find out if this was permissible for Americans or do I need to fly into Tehran with a booking agency?
Answer: As an American traveler in Iran you have no other way except taking part in a guided tour, then they apply for your visa and you will be permitted to enter Iran by land, sea or air.
You should fill in the visa form on http://www.uppersia.com/Iran-travel-services/iran-visa.html, it takes about 10 working days to get your reference number and two working days to collect your visa at the embassy by direct referral or one working week by posting the docs to the embassy. You can pay the visa fee in cash in Iran.
Check the cheapest Iran tours at http://www.uppersia.com/Iran-tours