Not exactly reading material but the DBT Skills Handbook is superbly helpful. It teaches you a lot of skills that once you learn are easily integrated into your life naturally. It's only $36 and easy to work through on your own without a therapist.
I also like Gary John Bishop's books.
I just read Polysecure by Jessica Fern and highly recommend it. The first half is a rundown of attachment theory and it’s the best, most helpful description of attachment that I’ve found as someone with avoidant tendencies. The second half gets into polyamory, still using attachment theory as a base to draw insights from.
At times it can kind of read like a “how to” guide moreso than an investigation into what it means to be poly, but there’s a lot of good material for ruminating over the latter as well.