Very, very carefully.
Seriously, one of the easiest ways of offending friends is criticizing the way they raise their children, even if you mean well.
I'm sure that you have only the best intentions for these children, but so do they. Or at least I hope so.
Being a parent is challenging in more ways that I would ever had though possible before I had my children. You try to do everything right, you have probably read a dozen books on the subject during the pregnancy and very likely you know the therory. Or many of them. (Your child didn't read the memo, so it will likely not play by the rules, btw.) Yet after a day of running after a toddler or juggling daycare, work and managing a household, you are simply beat.
Besides, you will be pestered by others who have different ideas about how a child should be raised, from family members to strangers on the street. If you doubt this, simply ask around what people think of sleep training, co-sleeping, breastfeeding and similar topics.
So how to get your message across? I see two ways.
You work in the field. You could occasionally drop a comment in a conversation about coming across some interesting reasearch on fostering early literacy, ask them for their opinion and experiences and hope that they take the bait. You might learn a few interesting aspects yourself that aren't in the papers you read. Don't pressure it if they don't. Do not make it a discussion about their way of raising their children, do not say "you really should do more...".
They know that you are a potential source of information and might tap into it at a later time. Don't burn bridges now, be open-minded.
Be the role model yourself. Be the example to watch for the child you live with and introduce books to the life of the other ones. Give books you think your friends will enjoy for Christmas and hope that they will find the time to read them so that their child can see it. And please make it light reading, they probably won't have the energy for "big literature" right now.
Be the "uncle" who picks good books or good magazines for the child in question and continue to do so over the years. I had an uncle like that myself and will be eternally grateful to him.
And calm down. Yes, early literacy is a factor on academic success. But not the only one. Love of learning, encouraging curiosity and fostering an atmosphere of "discovering the world" are important as well. Nothing is ultimately decided in the first years.
Your friends would perhaps benefit more from you babysitting for them (hint: let the children see you read if you find the time) than from more information on what they should do. A relaxed parent will do a better "job" and have the strength to do more of the "should do's" we all know.