This is the 9085th most frequent Japanese word.
溜め込ん
"溜め込ん" means "to accumulate" or "to hoard."
In this sentence, "溜め込ん" indicates the act of accumulating or keeping emotions internally without expressing them.
彼はいつも感情を溜め込んでしまいます。
He always ends up bottling up his emotions.
Here, "溜め込ん" refers to the act of collecting or storing items, in this case, books, in a location.
この本棚には多くの古い本を溜め込んであります。
This bookshelf has accumulated many old books.
"溜め込ん" in this context expresses the concept of gathering or holding stress internally, implying that it builds up over time.