Firstly, many, many years ago, I tried to read this book. In that edition they were instructing which colours to visualize in order to reborn in certain realms. But a footnote explained that they weren't too sure about one particular colour, whether by visualizing it you it would lead you to heaven or hell. I stopped reading at that instant! "That's not something to take a chance on!" I thought, " I think I'll try to do actions that will lead me to heaven rather than trying to remember to visualize a colour that I'm not sure will lead me anywhere but confusion."
I don't know much about Tibetan Buddhism, but whether I die or fall asleep, I feel it would be helpful to have the mind focused on my chosen meditation subject, knowing that if I direct my mind towards this, then good results will come from it. In this way, I will be relying on the Law of Kamma. Having practiced these methods in this life, I have seen beneficial results and have confidence that, if there is anything after death, then beneficial results will come from beneficial causes. This is rather than choosing a method I am not sure is true, and which even the authors seemed confused about.
I have great confidence in the Law of Kamma because I have seen it in my life that everything I think, say and do has a good result if it's beneficial, and if it's not, has an unbeneficial result. So, if I train my mind in my life to develop beneficial intentions, words and deeds, and practice beneficial meditation subjects, then it's more likely to come up at the moment of death as well. And if I die a swift death without much time to direct my thoughts anywhere, then it is taught that my habitual accumulated good Kamma will take care of me.