Your mind is what limits your dreams. Reading a book in a lucid dream is not a limit, it is an impossibility.
Of course, if you have read the book before and have a photographic memory, go ahead and read it, otherwise you are merely making up words. In dreams there is no information input, we are limited to what we know. I'm not sure if what Leo says is true, but I keep an open mind and will explore those possiblities. However, if the Higher Mind helps your dreams become more vivid and hands over most control to you, could it not take the control away and produce a dream so powerful that it forces you to believe it is real, at which time it could continue teaching you whatever lesson you need to learn. I doubt it would simply give up on you and release its connections. This is all assuming that there is a Higher Mind. Right now I only believe in the subconscious. Lastly, my beliefs about the 10% of brain "myth" is that we only use 10% of the power our brains have. I've seen people try to disprove the 10% "myth" by saying, "Have someone cut off 90% of your brain and see how you function". This is not a good point considering the power of your brain resides within all of the brain. Thats like saying, "Cut off 90% of your engine and see how well your car runs", It's not about using 10% of the parts, it's about using 10% of the power. A good example would be our cars. If we push the gas pedel down slightly, only a little of the power that it is capable of is used. I believe our brains are stuck in 1st gear.