Simplify the following text for a Grade 10 native English-speaking person.
Original: In 1946 he undertook the first of two expeditions into the Amazon jungle , the first of which produced a popular book , `` Along Native Trails '' ( Por Tierra De Indios -RCB- .
Simplified: In 1946 he undertook the first of two expeditions into the Amazon jungle , the first of which produced a popular book , ( Por Tierra De Indios -RCB- .