This is the 3523rd most frequent Arabic word.
وبعضها
"وبعضها" means "and some of it" or "and some of them" in English.
The word "وبعضها" is used here to refer to a specific subset of a previously mentioned group (the trees), appending new information about this subset.
الأشجار كانت جميلة، وبعضها كان مزهراً.
The trees were beautiful, and some of them were blooming.
In this sentence, "وبعضها" points to certain ideas within a general set of mentioned ideas, marking a distinction of novelty for these specific ones.
اقترحت عليهم أفكاراً، وبعضها كان جديداً تماماً.
I suggested ideas to them, and some of them were completely new.
Here, the term "وبعضها" indicates a select portion of the books described, adding qualitative description of those specific books.