Hey great work beej! I've read pro git and your guide is very good.
So in figure 5.4 you say we merge 2 commits into a new one and somehow both branches point to new commit. This will definitely confuse people new to git.
I'd say it's better to write we merge anotherBranch into someBranch and leave the former where it is. Same for the next merge.
In 9.4 there's no way reallinux/master points to same commit as master after the merge. It will still be where it was, one commit behind.