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. Book 4 was released in 2005, utterly doing away with the "every two years" that Martin had kept to beforehand, with an afterword promising that the next book would be released "next year". Put bluntly, that didn't happen. Fans were kept waiting until 2011 for Book 5, by which time the TV adaptation Game of Thrones had already begun. Martin was adamant that his books would stay ahead of the series, though, and promised his fans that he would do his utmost to get Books 6 and 7 out the door before their plots were revealed onscreen instead of onpage. In 2016 the show overtook the books, with fans still waiting for Book 6 ... and in likelihood then being faced with another long wait when it does eventually see the light of day before Book 7 is released.
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.
The Obernewtyn Chronicles by Isobelle Carmody: There was a decade between the fourth and fifth 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 finally turned up in 2013. One can only wonder how old we'll all be when books 4 through 7 arrive.
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.
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. It was ultimately released in 2014, with Book 8 following the next year.
Happened to the The Kingkiller Chronicle by Patrick Rothfuss. He absolutely promised the books would be finished in three volumes and the third book. There was three years between The Name of the Wind and Wise Man's Fear with no announcement for the third volume as of 2016.
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.
According to legend, the infamously sudden ending of the first book came about because Adams's editor literally called him one day and told him in no uncertain terms to finish the page he was on and hand in his manuscript, or else.
In his second autobiography, Stephen Fry recounts an occasion when Adams - a close friend of Fry - was visited by his agent because a project was long since overdue. When asked how the book was coming, Adams apparently replied with "Excellent! I've just written the first sentence!" before condemning that one sentence to the mercies of the Delete key. Adams was also, according to Fry, an appalling procrastinator, who could spend hours at a time adjusting inconsequential settings on his computer instead of writing.
Tamora Pierce can be prone to this. The first book in a planned series about Numair was set to release in Fall 2015. She did finally get a first draft done... in May 2016.
Philip Pullman's His Dark Materials books tend to eventually emerge rather a long time after they're announced. The Subtle Knife took two years to emerge after Northern Lights, and then The Amber Spyglass took almost four. He mentioned his intention to write a companion book to the trilogy called The Book Of Dust shortly after the publication of The Amber Spyglass in 2000. Seventeen years later the publication date - of the first part of what will now be a trilogy - was finally announced for later in the year. However, he did publish three small supplimentary pieces in the interim to keep us just about going: Lyra's Oxford, Once Upon A Time In The North and The Collectors.