In some ways, writing the book was the easy part. The hard part is making sure not to forget to thank everyone who helped in its development!
We had been talking about writing a book together for years, but there was always some reason why we didn’t get around to it. Finally, the opportunity coalesced and we couldn’t find a reason to put it off any longer.
I don’t know how you should understand the word coalesce in this context. Any thoughts?
I understand it as figurative language.
Literally, “coalesce” means to come together to form a united whole, like droplets of water coalescing into a puddle [Wiktionary]. Or you could say that four independent-minded musicians, with conflicting ideas about how to perform their music, coalesced into a quartet—meaning that they learned to play cooperatively, forming a whole, performing as if they shared one spirit.
Metaphorically, in the context of the book preface, it means that the opportunity to write the book didn’t exist until many things and people became ready at the right time, but eventually that did happen. The previous sentence mentions that there was “always some reason” why, in previous years, they didn’t write the book. That is, in previous years, some factors were always missing, which were needed to form an opportunity that the authors could act upon.
Unlike a literal coalescence, a bunch of independent factors needed for an opportunity don’t grow together or unite. The passage just asks you to think of it that way. Or perhaps the coalescing was more literal, if perhaps one person’s travels enabled them to make useful notes, which “attracted” an artist interested in converting the notes into drawings, which “attracted” a coauthor with the right qualiifications, and so on. In that case, there is more of a feeling that the elements of the opportunity are growing together rather than independently happening to become available at the same time.