Berengar's notes on the Beast is a quest item, and as such, it can neither be bought nor sold, only consulted. Abigail gives these notes to Geralt when he asks her about the beast.
"First night: 95 kg of muscle, height around 4,5 feet, fangs the length of a man's thumb — to be avoided. Shares characteristics with Alzur's Demon.
Second night: summons barghests with its howling, has the ability to 'disappear', probably by assuming spectral or spiritual form, can only be seen with the help of a potion.
Third night: highly resistant to iron and Signs, walked right over three Yrdens without noticing.
Fourth night: all features of Alzur's Demon confirmed. The Beast asks a question, those who answer correctly stand a chance of slaying it."