You read the same paragraph four times. You put the book down and pick up your phone. Ten minutes later you are thinking about your ex again. Low concentration is common when your attention is being repeatedly pulled back to something emotionally important.
Stop choosing books based on the version of you who could focus for an hour
Dense psychology, long memoirs and heavily structured self-help can be useful later. Right now they may simply give you another thing to abandon halfway through. Match the format to your current attention span.
Short chapters, illustrated pages, essays you can finish in one sitting and books you can open anywhere are often easier when your brain is restless.
Choose comfort before analysis if analysis is exhausting you
There is a time for attachment theory and deep reflection. There is also a time for reading something kind enough that your nervous system does not feel like it has been assigned homework.
The Boy, the Mole, the Fox and the Horse (affiliate link) on Amazon is a good fit for low-concentration periods because it is visual, easy to dip into and does not require you to follow a long argument from chapter to chapter.
Use reading as a replacement for the habit you actually want to interrupt
Put the book where the problem happens. Next to the sofa if evenings become scrolling. On the bedside table if you keep checking your ex before sleep. In your bag if lunch breaks are when you spiral. Friction matters: the alternative needs to be easier to reach than the habit you are trying to interrupt.
Try a ten-page rule instead of a reading goal
Tell yourself you only have to read ten pages. If you want to stop after that, stop. This removes the pressure to "get back into reading" and turns it into one small decision you can repeat.
If having several books available without keeping your phone in your hand would help, a Kindle Paperwhite (affiliate link) on Amazon can be a practical way to separate reading from the social apps and notifications that often hijack the same screen.
When should you move to deeper breakup books?
When you can read without every paragraph turning into a forensic comparison with your relationship. That is usually when books about attachment, boundaries, patterns and rebuilding start becoming more useful than overwhelming.
You do not need to learn everything about the breakup immediately. Sometimes the first useful job of a book is simply to keep you company for twenty minutes without asking anything from your ex.