This is the 588th most frequent Lithuanian word.
toks
"Such" or "so".
Here 'toks' is used as an adjective modifier to indicate the quality of friendliness in a person, expressing admiration or emphasis.
Jis yra toks draugiškas žmogus.
He is such a friendly person.
'Toks' is declined to 'tokia' to match the gender of 'knyga' (feminine) and emphasizes the high degree of interest the book provides.
Ši knyga yra tokia įdomi!
This book is so interesting!
Here 'toks' compares or describes the manner of acting, implying the speaker finds the behavior unusual or extraordinary.