nee - so sorry for the late reply!
My goodness, 18 and a half read of Cat in the Hat a day?!? I am nowhere near that!
You raised an interesting point about increasing the intensity, frequency and duration - so do more, more often and for longer...hmmm...some thoughts on this:
Intensity - reading a wide variety of books from easy to difficult, old favourites to unfamiliar, fiction to nonfiction etc. What type of books do you read? Do you reread some of them? Something that I am doing at the moment, at my daughter's request, is reading a book immediately after she reads it. I think she likes to listen to how I say the words, the expression, voices, etc., it allows her to focus on the pictures (on the page or in her head) and she will often ask questions about punctuation too. We generally only read 1 chapter a day, with lots of picture books thrown in to the mix - it sounds pretty pathetic compared to your goal of 15-20 chapters!
Frequency - you could go for the HSIYB approach where you read aloud 10 times a day. I keep books everywhere for this - obviously in the children's rooms, by my bed, the kitchen table, the sofa, every room in the house, my handbag, the car, the buggy, and we have a book bag for when we go to cafe's. I usually read while nursing the baby too. (the downside is that the house is very messy with books thrown everywhere!)
Duration - longer reading sessions, never missing a day
What age is your little boy now? It's wonderful to hear that his enunciation has improved! Are there particular sounds he has difficulty with? Is it the start or the end of words? The start or end of a sentences? Or is it overall clarity in general? Does it warrant an assessment by a speech and language therapist?
Have you tried videoing your little boy or getting him to speak in front of the mirror? Sometimes children are unaware of the movement of their mouth and that they can improve their speech by becoming more aware of this.
Are you familiar with the Handwriting Without Tears approach? They advocate that children write one or two letters in their best handwriting, rather than writing the same letter many times of variable quality. Maybe you could try that approach, where at certain times of the day, you focus on one sound you would like to address and play games involving words that relate to that sound.
Sophie loves to read our baby's very simple board books, one or two words per page. Maybe you could ask your little boy to read these books to a younger child, asking him to read slowly and clearly to teach the baby.
Tongue twisters and games like Telephone are great for helping children to realise the need to clearly enunciate to aid understanding. Also, verbal memory games that involve repetition, e.g. I went to the shop and I bought... etc.
Learning poems and songs off by heart - again, repetition helps child improve pronunciation.
Have you read Fowler's book or watched the DVD? You may get some other ideas there.
Just a thought - are there any issues with his palate or tongue (tongue tie)?