Proofreading's first rule - make the words REALLY BIG

The book I created for my research project came back from Blurb the other day. And it looks great.

But, but, but...
There are mistakes. Hideous blunders. SPELLING MISTAKES, for God's sake. Or at least typos.
Now I'm a grammar nazi, up to a point. I don't know enough about it to get things spot on, but I've read enough to have a good idea what looks wrong and what looks right.
Some of the lines are single spaced, when I wanted them all double spaced. Sometimes these two options are on adjacent pages! And I didn't spot it.

I figured out that it was the fault of the smaller image on the Mac screen. The defects went unnoticed despite the horrendous length of time I spent staring at the screen. But they screamed out at me when I saw them actual size.

Lucy once gave me a great piece of advice: test print EVERYTHING before the final edition. But the college was closed, every time I came in the Mac suite was bolted and barred, and anyway it would have cost a tenner...so I didn't. But I wish I had.