The main reason for the backlash against George R. R. Martin. The first three books of A Song of Ice and Fire were released in 1996, 1998 and 2000. The announced release date for the fourth book was pushed back several times, as the book grew longer, until Martin had no choice but to split it into two books. release date of the fifth book (the second part of what was supposed to be book 4) was also pushed back many times, and it was finally published nearly five years after the first projected date.
Abarat fans were told that Absolute Midnight would come out in 2004. It eventually came out... in 2011.
Seven years separated the first book of The Unicorn Chronicles from the second, and seven more years separated the second from the third. Some of the children it was initially marketed to have now graduated from college. Fortunately, the fourth book took only two years to come out.
Isobelle Carmody: There was a decade between the sequels books. Many of her younger fans have gone through high school and finished a degree in the intervening time when waiting.
J. K. Rowling intended to release a new Harry Potter book once a year, and was successful for the first few years in keeping this up, but ended up with a four-year gap between books 4 and 5, likely due to the production of the films starting during that time. The so-called "three-year summer" is considered to be a factor in the massive boom in Harry Potterfanfic that began around this time. The fifth book contains what may be a sideways reference to this, when Harry feels like summer has been dragging on forever, and is eager for something to actually, finally, happen.
Captain Underpants has suffered this. The first five books came out on a yearly basis (1997-2001). Books six and seven (a two-parter) both came out in 2003 (Although 2002 had a spin-off book and activity book). Book eight came out in 2006. Author Dav Pilkey then took a couple years off writing to take care of his ill father, and after two spin-off books in 2010 and 2011 respectively, book nine and book ten (a two-parter) both came out in 2012 and 2013 respectively, and it has been promised the series will return to a yearly basis.
Scott Lynch, author of the Gentleman Bastard series, falls into this. The third book's release date has been pushed back multiple times. To give a sense of scale, the second book was released in July 2007, and the third book has slid to the end of 2012.
Strange Fate, the tenth book in the Night World series by LJ Smith, was originally slated to come out in the summer of 1998. This book was meant to introduce the final Wild Power and was slated to be the second-to-last book in the series. As of October 2012, Smith is still writing Strange Fate, making Amazon's pre-orders for it in 2011 both confusing and frustrating. What makes this more frustrating was the finale of the series originally intended to include the Apocalypse, in which all four of the Wild Powers came together on January 1, 2000, to fight evil. Smith intended to finish the series before 2000 to make this scenario more plausible. When the year 2000 came and went in real time, Smith made the decision to completely scrap that storyline and rewrite it so that it took place after the Apocalypse. What's more, Smith had decided that because of the delays that Strange Fatewill be the final book in the Night World series, canceling the eleventh book idea that was meant to wrap up the series.
Terry Pratchett, is accused of this, but it's not really happening. His books are coming out on schedule; it's just that the schedule isn't what fans think it is. Fans tend to take any reference to a title he's toying with as being "the next book", only for the next book to turn out to be something completely different. This means Raising Taxes and Scouting For Trolls have been subject to "schedule slip", while he continues to produce at least one book a year. Producing books at this rate (and even more so in the middle of Pratchett's career, when 3 books was a normal year) means that at any given time, the book he's in the process of writing is unlikely to be the next one that actually appears in stores- that will have been written some time ago.
The Assassin King, Book 6 of the Symphony of Ages, was released in 2006. After seven years without any further word on the series, Book 7 is finally rumored to be scheduled for an August, 2013 release.
The Power of Five: One book was released a year between 2005 and 2008 (albeit with a few months' difference). As of April 2011, Anthony Horowitz wass about to start Oblivion and was aiming for a 200,000 word count, stating that it could be "years" before it materialises.
Oblivion was finished and edited as of May 2012, and was finally released on October 4, 2012.
Douglas Adams was famous for this, as seen in the page quote. Neil Gaiman's biography, Don't Panic, includes the text of a rather panicky sales kit for So Long, and Thanks for All the Fish, compiled long after the deadlines, and shortly before Adams had basically been locked in a hotel room by the publishers and told not to come out until he'd written something.