The magical journals were developed in 1916 and 1917 as part of research and development efforts during the Great War. Originally designed for rapid communication between various military and support arms, they were of course in high demand following the War.
Bound For Perdition gets into the journal construction in more detail, thanks to the work of
Lynet Alder and
Reggie Hollis.
Journals require a significant investment of handmade materials and labour that cannot be done by machine, as well as specific magical materia and enchantments. As a result, the cost is significant - somewhere around twice the cost of a higher end computer. On the other hand, they're tremendously useful, and so by the mid-1920s, many people who can afford the expense either have one, or will as soon as one is available. And by the later 20s, various advances have reduced the cost somewhat and made them somewhat more available due to having more people able to make them.
The magic of the journals relies on the qualities of the paper, the book binding, and the enchantments placed on the final book. By the 1920s, these are handled by a small number of specialists, working together to create the finished pieces to the necessary standard.
The standard size for a journal is a "Council Octavo", a 6" by 9" size, bound with 14 signatures of 14 pages each (7 sheets), for 196 pages total. Each page holds a separate conversation, so if you want to maintain more conversations, you need a new journal. Charms and other enchantments help with indexing (think of automatic bookmarks that let you jump to the most recent messages, as well as skim back to older conversations.)
Because this can get clunky, people who maintain long-running conversations may copy them magically to a separate volume where more detailed indexing charms can be applied. Both parts of the conversation are visible, which also makes the journal a possible security risk in some situations.
There are of course ways to limit who can contact you via journal, but any conversations you have had in the journal remain visible to the other party, even if you cut off contact.
