Forgive me, I am not certain what you mean by 'undercover regions'. Lady Mary Wortley Montagu's travel writings on Turkey and other countries are used as primary sources. Her relish in appealing to her audiences means her writing isn't always reliable as verbatim records of events, but is insightful for us for a range of reasons. Including for understanding then prevailing views of some English people of the Occident and Orient.

There's lots of sources that could be considered travelogues, and some have of course attracted more scholarly attention than others. A few that I'd particularly recommend:

• For the early medieval north (Vikings, Slavs, Bulgars, Khazars, etc., from an Islamic perspective), there's a great collection in Ibn Fadlan, et al., Ibn Fadlan and the Land of Darkness (Penguin, 2012) [$10-12].

• I'd also recommend reading up on two Norse (aka "Viking") traders who visited Anglo-Saxon kings in England: Othere and Wulfstan.

• For more recent accounts (which are better covered by scholars, but still very interesting), I'd look at Olaudah Equiano's autobiography. Equiano was sold into slavery from Africa, but eventually purchased his freedom and became an abolitionist working against slavery in London. In between, he traveled through the Caribbean, the Mediterranean, the Atlantic, and the Arctic. Here's an online collection of sources on Olaudah Equiano.

• And another brilliant narrative of the early modern slave trade is Robert Harms' The Diligent, which uses an officer's journal to track the journey of a slave ship in 1731. Harms is a great writer, and he's provides an excellent example of what all we can learn from a travelogue like a ship's log.

Charles Darwin's diary from his journeying with the Beagle might also interest you.

Are there any others that histroins use as primary sources for undercover regions.

Hard to understand what you are saying. But I am working on neglected travelogues from neglected regions, but for that very reason, they are not considered "important." I'm hoping to make them important.

I don't know if these count as "undercover," but:

The Discovery and Conquest of Peru

Our First Ambassador to China

Or J. Barrow's Travels in China which recounts the same trip, but with less formality and more racism: "The old eunuch in charge had the impudence to claim that this clock was the workmanship of a Chinese" upon being shown a clock which played English tunes and had the clockmaker's address in London written on the dial.

Pretty sure they're all on Google Books.