Alas, this was a rather easy list to compile by just searching past reviews for the words "disappointed" or "disappointment".

Sense and Sensibility by Jane Austen
How the Mind Works by Steven Pinker
Olympos by Dan Simmons
Fanny Kemble: A Reluctant Celebrity by Rebecca Jenkins
Fermat’s Last Theorem by Simon Singh
Pyongyang by Guy Delisle
The Thackery T. Lambshead Pocket Guide to Eccentric & Discredited Diseases
The Alphabet: unraveling [sic] the mystery of the alphabet from A to Z, by David Sacks
Like a Velvet Glove Cast in Iron by Daniel Clowes
Back to the Vortex by J. Shaun Lyon
Doctor Who and the Sea Devils by Malcolm Hulke
Doctor Who – Ghost Light by Marc Platt
1610 by Mary Gentle
Blood Upon the Rose: Easter 1916: the Rebellion That Set Ireland Free by Gerry Hunt

Day 01 – Your favourite series of books (with more than 3 in the series)
Day 02 – A book that you wish more people had read
Day 03 – Your favorite recent book
Day 04 – Your favorite book ever
Day 05 – A book you hate
Day 06 – Your favourite writer
Day 07 – A writer you don’t like
Day 08 – Your favourite work in translation
Day 09 – Best scene ever
Day 10 – A book you thought you wouldn’t like but ended up loving
Day 11 – A book that disappointed you
Day 12 – An book you’ve read more than twice
Day 13 – Favorite childhood book
Day 14 – Favorite male character
Day 15 – Favorite female character
Day 16 – Your guilty pleasure book
Day 17 – Favorite trilogy or tetralogy
Day 18 – Favorite book cover
Day 19 – Best ensemble of characters in a book
Day 20 – Favorite kiss or love scene
Day 21 – Favorite fictional romantic relationship
Day 22 – Favorite ending/climax
Day 23 – Most annoying character
Day 24 – Best quote
Day 25 – A book you plan on reading
Day 26 – OMG WTF? plot
Day 27 – Favourite non-mainstream writer
Day 28 – First book obsession
Day 29 – Current book obsession
Day 30 – Saddest character death