I suppose it all depends on how often you are doing science and what books you are looking at. I am planning first grade also. We have science planned for once a week. If many of your books are short picture books, then you may be able to get through most/all of them, but if they are chapter books perhaps not so many. Also, do you plan to do experiments that would take your lesson time?
Once you have decided how much time you are spending on science each week/term you may have a better idea of how much reading you can accomplish. I understand the temptation to not miss out on any great reading, but it may be better to leave ds wanting more and asking ‘Can we read more about…?’ Especially in the younger years.
Just my two cents,
Vanessa