I was reading a book COMPLETELY NOT RELATED TO ASTROLOGY WHATSOEVER , ACCIDENTLY read something which said such and such a thing is an astrological sign Does that mean that I have learnt astrology and committed disbelief ?
The opposite of disbelief is belief.
So do you believe what you've read is true? If yes your belief is the opposite of what a Muslim should believe in and therefore it will be regarded as disbelief or maybe shirk if you considered both Allah's rules and what you've learnt about the astrological signs as (parts of) the truth.
It is rather frowned upon to read what may cause doubts in your religion (faith) or shirk or disbelief. Some scholars would consider this even haram for a person who is not strong in his/her faith. Therefore it is recommended to seek refuge by Allah and repent from what you've did no matter if the answer of the above question was "yes" or "no". However if it was "yes" you should even repent more sincerely and avoid doing this again.