Enter T9, one of the predictive text apps for phones with 9 digit keypads. Ironically, the logic behind T9 is to move into common usage, the words you use most often when sending a text. However, it obviously doesn't move fast enough for our "power" generation!! This is how the expression "cool" became "book."
Apparently the keys 2665 can be either "book" or "cool." Logically "book" comes up first, as a suggested word in T9. Rather than taking the time to hit another key, thereby moving to the next suggestion, which would be "cool," today's (cool) users realized that they would miss a beat and decided to stay with the original word offered, which was "book."
Now we were already dealing with a slang term "cool" meaning trendy, which has now morphed to an even slanger (can I say this) term, "book." The verbal logic is not there. However the tech logic is!! Ironically, book has become cool at a time when books (hard copy) are becoming less cool. The tech world may be out of touch with the real world. Then again, the tech world may be creating a new world, with it's own language and logic.
Here is another word shift in the making, 4663 - good, home, gone. In the future, will "He's home;" really mean, "He's gone." Time will tell.
Have a book day!!