I do take a notebook with me - usually a Write-in-the-rain one to scribble or sketch on even if the weather turns against me - but I tend not to carry books into the woods. The reason? Well, once there, there's plenty to do, what with setting up camp, sourcing fuel and water, working out what's on the menu for that day and so on, so that time spent perusing a book, particularly a reference or i.d. book is short. Occasionally, I'll throw in the laminated swivel card things produced by the Woodland Trust - one on trees, the other on plants. They are basic guides, I think designed for children, but they can be useful, if not particularly detailed.
Once 'camp' is established, I like to try and practise my craft skills - developing the few I have or trying variations. If wandering around and uncertain about something, be it plant, tree or track, I'll take a picture of it on my 'phone, make any appropriate notes about its situation and then dive into the reference books when back at the house.
Having said all that, I do like a bit of fiction with me: it relaxes body and soul and helps me sleep well at the end of the day.