This is the 2578th most frequent Arabic word.
البالغة
"البالغة" means "adult" or "mature" in English.
In this sentence, 'البالغة' is used as an adjective to describe the complexity of the novel, indicating 'reaching a high degree of complexity'.
أعجبتني الرواية البالغة التعقيد التي قرأتها الأسبوع الماضي.
I admired the highly complex novel I read last week.
Here, 'بالغةً' acts as an adverb emphasizing the degree of 'higher than expected' costs.
تكاليف الرحلة الأخيرة كانت بالغةً أكثر من المتوقع.
The costs of the last trip were higher than expected.
In this instance, 'بالغة' qualifies 'الأهمية' (importance), conveying 'significant' or 'critical' importance to the outcome.