He put in a few extra too! This was a great unplanned every-day-life-as-learning activity, à la Waldorf style. After putting in all of these pegs, and a few more, E had really improved his technique, realizing how to hold the pegs in order to put them in.
I also recently finished this book sling for the playroom (a.k.a. dining room) from this tutorial. With the books displayed this way, the cover art is visible to E, so he can see which book he's picking up. Hopefully the book sling decreases the pull-every-book-off-the-shelf-to-find-one mess and frustration.
The plan is to only keep about four books in it at once and to rotate them periodically. It's a bit too tall for the small boardbooks we have, so those are kept somewhere else (not as neatly...).