I can't give you a book, but perhaps my experience in the matter will be of some insight to you? I'm pretty big on self-help because I can't afford or find a good therapist. I've had to learn, the hard way in many cases, how to best treat the people I want to keep in my life. You've already realized that clingyness is doing the opposite of what you intend, and that's a great first step! Now the harder part, curbing the habit...

Also worth a note before I get into it that this is just what works for me, there may be a better process out there for you.

After identifying the issue, the next step was to spot in the moment what I was doing to be clingy and defeat it with logical thoughts. For example, if the person I was attached to wanted to hang out with a friend of his instead of me that day and I started to get jealous, immediately I would say to myself "that is not fair of you, he has to be allowed to have time away from you, you do not own or control him". Maybe more difficult is issue you mentioned, where you're expecting a certain amount of communication on his end and he's not doing so. For that one there's a few steps I like to do. Firstly, we need to establish what is a realistic amount of communication. Every second of every waking moment of course is not a healthy relationship. Start to adjust your expectations to a realistic amount. This will also vary depending on your specific situation with each person because some people require more space than others to not feel smothered, so it will take a bit of analysis on your part to find what is appropriate. If he's willing, I even like to establish a verbal boundary so that I have a good idea of what is reasonable. Second, defeat the clingy thoughts with as much logic as possible. "He must be busy, I'll go do something else for now then." Such things like that. As hard as it is, you gotta give him the benefit of the doubt that he's not just ignoring you or it won't work out, no relationship can work without a solid level of trust. But of course, if he's constantly avoiding you then that's not a good sign either. Don't let yourself be so attached that you get walked all over either.

Then there's moments when it really gets tough... Maybe you've asked something important and he's taking his sweet time responding, or he's just in general taking hours to respond to something. The only thing I found that helps here is distraction, go read a book or watch a movie/show or some other hobby you might have. If all you do is hyperfocus on the issue and end up pressuring for a response or expressing how upset you are, it just makes it so much worse.

Another small exercise I have tried is to not respond immediately to text messages myself. Arguably this has helped me the most because it has trained my brain to realize responses do not need to be immediate, I can take my time and it's ok if he takes his. This was also very hard to put into practice, and I still fail it when it's an issue of needing comfort from a response, but overall it has gone well.

And I do realize it all sounds easier said than done, and I'm not gonna lie and say I'm perfect at it, but it has made me so much more reasonable. Just yesterday was the first time in a long while that I slipped because I was having an already bad day. I'm proud of myself for making that much progress. Remember to be kind to yourself during this process, don't focus on how much left you have to do and instead focus on any positives.