This is still ongoing, and the glitch freebies are in a definite state of flux, with some actually seeming to be added.
For example, there was only one Aaron Elkins freebie yesterday when I searched directly on his name, but then another one showed up today. So it may be worth your time to check back on a favourite author/category.
Also, I just had a look at the iTunes vs iBooks versions of the store, and I'd forgotten how hard it is to find things on iTunes.
So an additional tip:
Use the Top Charts sidebar feature at the bottom left of the main Books within iTunes page, and click on it to take you to the actual top charts, which you can then filter by category, and then to free.
It'll give you a lot more authors/books than you can see directly via keyword search on just Penguin Group. Unfortunately a lot of the really good non-fiction freebies don't show up that way, as well as the more obscure literary fiction types.
Here's a couple more lists of authors I was checking on previously, who yielded up freebies in the store search listings, if it helps anyone. I may have misspelt some names or shortened them if the first name was distinctive enough that it could be used as the sole search term:
These were taken from the kids/YA category
Mostly sf/fantasy & cozy mysteries with some literary fiction: