But here's my question: when did publishers start using the more horizontal format?
If you're familiar with strip reprint books from just a few years earlier than these, you might recall that they pretty much all looked about the same size as the Heathcliff book. Regardless of the strip's format, publishers would pay someone to basically cut up the comic strips (probably photostats, not the originals) and reposition all of the panels so that they worked for the vertical layout. Here are two more reprint books I have from 1965 and '68...
The panels of each Peanuts strip have been stacked to form something more square-ish, and then stacked again so there are two strips on each page. The Pogo strips was reconfigured entirely so there are six panels on a page; my understanding was that Walt Kelly himself reworked many of the strips and redrew portions so it fit the format better than his original horizontal layouts. These are hardly unusual; I can recall my father's old reprint books from his college days were all in the more vertical format regardless if they featured Snoopy, Andy Capp, or Captain Klutz. In fact, I never recalled seeing a horizontal strip collection like that before Garfield.
It turns out that it was, in fact, Jim Davis who pioneered the horizontal books! The Wikipedia entry for Garfield at Large reads as follows...
This book introduced the "Garfield Format" to the comic book market. Prior to its publication, comic strip compilations were originally formatted like a standard paperback book with the panels running down the page. Jim Davis, Garfield's author, disliked the idea and coerced Ballantine to print the strips from left to right, as they would have appeared in the newspaper. This resulted in the final product being shorter from top to bottom and much wider from side to side than the average paperback book.Garfield debuted in newspapers in 1978 and was almost an overnight success and was running in over 800 papers by 1980. So when Ballatine was putting together the first Garfield collection that year, Davis had already amassed a good deal of clout -- already pulling in $15 million in merchandising. That kind of money (especially in 1980 dollars -- it's roughly the equivalent of $45 million today) carries with it a lot of weight, so Davis probably didn't have to do all that much convincing.
Not surprisingly, other strips followed suit and the horizontal format became more common. (Amusingly, I have a later Heathcliff book that uses that format, placing two of the vertical strips side by side so they fit on the wider page.) Although as many books include the larger Sunday strips as well, the precise dimensions of the "Garfield format" have grown appreciably taller. Though the books still remain wider than they are tall.
For as much as Davis is often dismissed as crassly catering to commercialization, I have to give him a lot of props for pushing forward a book format based on a form of artistic integrity!