Mint Error News is a decent free online magazine. Loaded with good pics and a LOT of very unusual errors - the kind that many collectors may not even know exist.
I was reading the 2006 special issue today -
Get it Here
240 pages, it's a 'best of' issue - I was checking out several good articles (one on specific gravity and one on two-headed / two-tailed errors). It's worth a read IMO