First to the above poster, EK bio book is the best book out there and its very clear. As to you dreamer67, you need to read more clearly and carefully. Each sentence of EK book is very valuable because they are very condence.
The answer is substances can reach ER lumen without crossing a membrane simply because the ER lumen is extended from the cell membrane to the cytoplasm to get along the nuclear envelope. In other words, the ER lumen is contiguous or in actual contant with the plasma membrane and therefore substances that cross the cell membrane can just get into the ER lumen via endocytosis without crossing another membrane since the cell membrane and ER lumen are connected. Read in top of page 75; it says, "In my places, the ER is contiguous with the cell membrane and the nuclear envelope. The ER lumen is contigous in places with the space between the double bilayer of the nuclear envelope." This is exactly what I said before. So in short, there is only one membrane to cross, which is the cell membrane !