I would direct my lucidity to relaxing and getting a normal sleep if that was what I wanted. But, it could be that, at the moment, you feel the mental workout is more important than getting more restful sleep; if the book was negatively impacting you, it is important that you direct your awake time so you have less negative impact; you cannot have a stressful day where it does not impact your night; they are connected (just like the opposite, workout night means feeling less rested during the day). One think I try to remember is that our unconscious is working on "balance" so letting it do its thing will probably work out for the best for us as we can't control the unconscious (because we can't know it until it become conscious). You don't have to tell yourself, from your head, how to balance and ride a bike; your body and automatic processes take care of that. Relax and know your unconscious and conscious know how to work together and if you are a little less rested feeling when you wake tomorrow, tomorrow will go a little better because you won't be as stressed from reading the book, etc.