How useful are the book chapters since they are not even indexed by Pubmed, the books are expensive to many scientists to purchase and thus cited rarely?

What is the best way to make a book chapter more useful since they are not publicly available and not indexed in pubmed?

Many of us have had less than satisfactory experiences in writing book chapters as part of a themed volume.

These books, when published, are expensive thus inaccessible to many scientists; therefore, they are cited and used infrequently through lack of availability.

I therefore, welcome suggestions on how to approach this issue.