The actual words an author chooses will have a HUGE impact on who can and cannot read their books.
The basics of this comes down to a concept in literacy instruction we call "fluency". This is usually a score of some kind or the other that tells an educator or parent how well a student can read a given piece of text. Fluency is based on time and the number of errors. If you can read quickly and not stumble or miss words, your fluency score will be high. Low fluency is, of course, the opposite.
The trick here is that fluency isn't static.
You may have, for example, a very high fluency when you read a Harry Potter book and a very low fluency when you pick up Moby Dick. Understanding who your readers are will then require that you have some basic knowledge of this concept. You may think it's cool to beef up your reader's vocabulary but, more than likely, you're actually alienating a large percentage of your readers. You can consider all of this if you have readers at home as well. By understanding the three categories fluency gets divided into you can shape your own writing and the books your children are reading to best suit their skills.
The first category is called "frustrational" and, as you can guess, your kids/audience aren't going to like reading much if you're pushing them into this territory. A frustrational reader is going to be making a word error more than about 5 out of every hundred words while they read and if you ask them some basic questions about what they read they'll only get about 70% of them right. Hand one of your children a frustrational book and you might be turning them off to reading for a very, very long time. Does this mean you should avoid these books at all costs? No. They're perfect books for you to read to them just so long as you take time to pause and talk through difficult words and check in every so often to make sure they "get it."
The next level is called "instructional" and it can get a little tricky. A reader with an instructional level book will have about 95-96% accuracy with words and will be able to recall information about 80% of the time. An instructional book is a great book for a reader to read-aloud to you so you can support them through their infrequent struggles. The trick with instructional books is that they can easily become frustrational. If, for example, a reader is tired and you insist they should keep reading (or, even worse, you insist they SHOULD be able to read it) it can quickly slip into frustrational and could put the skids on their enjoyment of reading. If you're reading with a child and the errors start to creep in, just slip in a bookmark and try again another day or suggest a slightly less difficult book. Pushing forward could actually turn your kid in a bad direction.
The last level, "independent" is your reader's sweet spot.
They only miss about one word out of every hundred and can pretty much nail any questions you ask them. These are the books that will make any kid, no matter their ability level, learn to enjoy (if not love) reading.
So, with all this in mind, do you need to be giving your child fluency assessments at home just so you can pick the right book at the bookstore? Of course not, but you should be paying attention to the books your child does read and be asking your child's teachers about fluency. Because your child, as much fun as it is, might not be ready for Harry Potter. Or the Hunger Games. Or whatever. Pushing your kids to read the books YOU want them too (or hope they will) won't help. Let your kids read what they want and what they enjoy even if you think they could be doing better. If you want to push, do so by reading them stories and modeling for them how your brain works when you get stuck.
What do you do when you hit a word that you don't understand?
What do you do when you miss an important detail?