There are a lot of variables. Mostly, how much power do you need? and what weight penalty (if there even is one) are you willing to trade for the inconvenience of dealing with solar. I would calculate how many hours of reading I would realistically do on a trip, then make a ballpark estimate of how many hours in one charge of the device, then determine the amp-hour rating of the devices battery. Multiply the rating by the number of charges to get a ballpark estimate of the size battery pack you'll need. You might be able to get away with a small one. You may already know how to calculate all that, so sorry if you do.
FYI, for my camera I just carry spare batteries. Also, I seem to bring a book on every 2 or 3 week trip and struggle to read even 20 pages. Travelling, fishing, camp chores and cooking take up all the time. I don't seem to have a moment to spare for some reason when I'm out on a trip.
Mark