For several decades now it's been well nigh impossible to avoid predictions about the demise of the printed book. In addition to all the hand-wringing about ebooks eventually replacing printed books altogether, there is the usual moaning and groaning about all those other things that steal precious minutes away from reading: TV, video games, the Internet, (insert here your own personal villain). Much of this is targeted at what all this means for "the younger generation." If kids today aren't reading, for whatever reason, from whence will arise the next generation of book collectors?
Rather, focus on what the child knows, can relate to, and has an interest in: poop, for example. Or just about any other bodily function. Books about bugs also would make for a nice beginning book collection--the higher the bug ranks on your child's personal icky-ness meter, the better. Very young children, especially, have an insatiable curiosity about themselves, adults, the natural world, which curiosity can readily be put to good use creating a new generation of book collectors.
To paraphrase Gordon Geckko: Gross...is good. Gross is right. Gross works!
[P.S. Due to prior commitments this Sunday, this is appearing a couple of days early.]