Answer
In the name of Allah my dear son, know that the Shi'a do not have any book that is entirely (100%) authentic except for the Noble Qur'an, which was revealed to the Holy Prophet (PBUH) and is preserved between the two covers of the book held by all Muslims.
Yes, there are books considered reliable, meaning that their authors were scholars and trustworthy narrators who compiled their works with honesty and integrity, documenting the transmission chains they included. We believe in the authenticity of these books regarding their attribution of narrations to the individuals cited by their authors. However, the chain of transmission of a narration must be evaluated by the scholar investigating it.
For this reason, we believe that Al-Kafi and similar books contain authentic narrations that must be accepted as being attributed to the Infallibles (PBUT). At the same time, there are narrations that we may not consider reliable due to weak chains of transmission, leaving their validity to be judged by those qualified in this field.
It is important to understand that the reliability or weakness of a chain of transmission is a matter of scholarly discretion. A narration may be deemed authentic and reliable by one scholar while being considered unreliable by another.
It is also worth noting that narrations that are not deemed reliable by virtue of their presence in these esteemed books are sometimes used by jurists under specific principles discussed in the books of jurisprudence.
The esteemed books, such as the Four Books form the foundation of Muhammadan, Alawite, Ja'fari Islam. Allah is the guide.