Hadith (Ahaadith) are narrations of Prophet Mohammed.
For a Sunni any doubt on any hadith from those six books are an act of blasphemy. That is the reason Sunni prefix the word 'Sahih' (Authentic) to those books, among those six book (Siha Sitta); Sahih Bukhari and Sahih Muslim are two ultra-correct books next only to Holy Quran or on par.
Shia are of opinion that since hadith are narrated by a chain of narrators who are fallible people bound to make mistakes no matter even if un-intentional, therefore, no hadith could be called 100% authentic (Sahih), thus, Shia don't prefix the names of books of hadith with word 'Sahih' (Authentic), because no matter how small but there are chances of error.
One major difference between Shia collection and Sunni collection are Sunni judge the person as right or wrong, if he/she is right; what ever he/she said is authenic. Shia rather check whether his/her hadith is right or wrong, since all people are fallible except Prophet Mohammed (saw), Fatima Zahra (as) and 12 Imams (as).
Shia have another set of books which are authentic but never ever 100% authentic since the chain of narration are taken from fallible people. There are four major books of Shia Hadith which are least in error, they are called 'Kutub al Arba' (Four books).
Four Major Shia Hadith book (Kutub al Arba) are:
1) Al Kafi (The Sufficient) by Mohammed bin Yaqub Kulaini, it has 15176 narrations/ Ahaadith.
2) Man La Yahdruhu Al Faqih (I am my own Jurist) by Shaikh Saduq Mohammed bin Ali, it has 9044 narrations/ Ahaadith
3) Tahdiib Al Ahkaam (The Refinement of Laws) by Shaikh Abu Ja'far Tusi, it has 13590 narrations/ Ahaadith
4) Al Istibsaar (Foresight) by Shaikh Abu Ja'far Tusi, it has 5511 narrations/ Ahaadith
Other Shia Hadith books are:
5) Bihar Al Anwaar (Ocean of Luminescence)
6) Wasaail Ush Shia (Details for Shia)
7) Haqq Al Yaqin (Reality of Certainty)
8) Ain al Hayaat (Essence of Life)
9) Kitab Sulaym bin Qays (Book of Sulaym bin Qays)
10) Sahifa Sajjadiya (Psalms of Sajjad/ 4th Imam), it is a prayer book.
11) Nahjul Balagha (Peak of Eloquence), compiled by Sayyed Radi, the book is collections of sermons, letters and quotations of 1st Imam Ali bin Abu Talib (as)
It is a Shia Islamic belief based on several Hadith found in above mentioned books, that there are several other books which are always under possession of current Imam, these books are never in possessions of people. These books include hand written Qur'an by Imam Ali Murtada, Jafr, Jabr and Mushaf Fatima (Codex of Fatima). Apart from these books current Imam also possesses original Suhuf Ibrahim (Scrolls of Abraham), Tauraat (Torah), Zabuur (Psalms), Injeel (Gospel) and original revealed books of every prophet of past times.
Oh Allah! Peace and Salutation be upon Mohammed and House-hold of Mohammed.
It is a Shia Islamic belief based on several Hadith found in above mentioned books, that there are several other books which are always under possession of current Imam, these books are never in possessions of people. These books include hand written Qur'an by Imam Ali Murtada, Jafr, Jabr and Mushaf Fatima (Codex of Fatima). Apart from these books current Imam also possesses original Suhuf Ibrahim (Scrolls of Abraham), Tauraat (Torah), Zabuur (Psalms), Injeel (Gospel) and original revealed books of every prophet of past times.
Oh Allah! Peace and Salutation be upon Mohammed and House-hold of Mohammed.