A book I've read states that けど is a 接続詞 (conjunction).
However, another book I had read states that it is a 助詞 (particle).
Who is right?
けど is a conjunction.It is not a particle. In conjunction with… …と関連［接続］して； …と合同［連絡］して, …とともに. and けど is also 接助・終助.
adverbial particles 副詞的小詞 《on, in, out, over, off など》.