In my humble opinion, children should be exposed to the Bible at an early age. Its the basis of morality. To this end, they can be taken to Bible school for a firm grounding on the subject.
Haha, I can relate to you there because I too read encyclopedias and even at one point started reading the dictionary. It's good to remember that reading in itself is inherently valuable, so almost anything the child wants to read should be encouraged. At younger ages, I find books that either encourage imagination and creative thinking or teach some kind of lesson the child can understand and grow from through the story are especially valuable.
Kids generally are inquisitive. They are curious to know the impossibility. I remember when I was in my grade two. I was around 9 to 10 years old. My late father had brought me to our municipal library. I saw a lot of books inside the crystal cabinet. The librarian assisted me to what books I want to read. Since at the time, the common references or books inside our library is the Encyclopedia Britannica. I pulled three volumes and kept on turning the pages, giving preference to the illustrations or pictures or drawing. I went home borrowing three volumes of the said books. There my love for books and my love for writing got started to develop.
If the child wants to read what they want, you should let them but not force them. They learn a lot more when they want to read than when they HAVE to. Reading encyclopedias, dictionaries or history books are good for them. They won't understand the crude things but they will gain knowledge from other information. I suggest starting them with more basic and easier books to introduce them to reading, though. It's to make it more fun and give them something they know about to read.
I understand your point but as a parent you are the best judge what is good for your child. I always bought what made her interested in the book shops and most of them were either pictorials with big pictures or cartoon books. After the age of 6 her interest changed and she wanted to buy books with short stories and related pictures.
I think children should be aloud to read what ever interests them. If they are interested the more likely it will be that they will finish the book. To be honest I never liked reading but there was a series of books I would gravitate towards to and actually read them all just because they were fun. If a teacher made me read a certain book I disliked I would usually skim through it.
You should always consider your children's age while buying books. A toddler should be given a book that largely contains pictures and few text. A 3-5 years children should have books that tells stories in simple words. Children in the age group of 6-9 should be given books that tells about science, history, people. The children who are between 10-13 are voracious readers, they should be given as much books as possible.