They have to allocate time in advance. The way in which a medical reader expects an X-ray to appear on a finished page is very different from what the geologist expects from a landscape illustration to one of his or her books. The difference between the ways in which a supplier handles books and journals comes down to the different schedules required. Journals have fixed schedules and recur in accordance with a regular timetable. Books can usually be fitted in.
Suppliers charge less page by page for book production than they do for journal production.
Different typesetters tend to specialise in one type of publishing or the other. In all cases a relationship where each party knows each other's procedures is of great advantage particularly where the publishing party is relatively small. Suppliers that can handle the whole process are to be sought in these circumstances. Larger publishers tend to work with a different printer from their typesetter and there are also cost advantages in working with a paper merchant to buy the most appropriate paper rather than using the stock bought in and regularly used by the printer. If different suppliers are used for different stages in the publishing process it is crucial that they fit together well.