It is the time period between purchasing a book and the time when you actually start reading that book.
I started reading The Google Story after 15 days of its purchase. Those 15 days I spent only in preparing myself to start getting into offline reading, all over again.
The main difficulty that I was facing is that should I calculate the days I took to complete the book as the three weeks plus some days since I started reading it or should it also include starting 15 days when I kept thinking of touching it :roll:.
Another example that came to my mind is that of FPS which I had completed in just 7 point some days without any Reading Period Lag because I had borrowed it from a friend. Therefore, external environment factors also affect the Reading Period Lag.