I have a few shelves of books by Idriess. You look along them and notice that most of the titles on the spines of these tomes - including the author's name (invariably Idriess, or Ion L. Idriess) and publisher (Angus and Roberson, or A & R. or A & R Ltd, except of cause for the 1927 Madman's Island, which has Cornstalk Company) - go across the spine (transverse) and that some go down the spine (that is, they are read from top to bottom) and that just a very few read the other way - that is from bottom to top. So, most of these titles you can read with your head straight, others (the top to bottom ones) you may tilt your head to the right to read easily, but the latter you have to tilt your head the other way, to the left, which is a little jolting and mildly annoying. But why has A and R arranged it so?
You will notice that almost all of Idriess's pre-war and war books have transverse titles, as indeed have post-war titles until about 1955, after which top-to-bottom titles predominate. Some transverse titles, at least on the dust jackets, have a variation in that the titles are angled rather than straight across. Most notably these are The Cattle King and Tracks of Destiny, and to some extent The Silent Service (1944).
But the book titles I find that are somewhat jarringly different are the three that read from bottom to top, namely, Onward Australia (1944), Gems from Idriess (1949) and Across the Nullarbor - DJ only - not the hard cover (1951). Why are these three different? It may have been that A and R want these to stand out and be different. Wikipedia notes that the USA, Commonwealth countries, Scandinavian countries and Holland have a (more or less) standard title orientation of top-to-bottom - if the title is not transverse - and indeed, top-to-bottom seems to have superseded transverse as the norm in more recent times (post 1950's). But central European countries (France, Germany, Russia, etc) and others have adopted bottom-to-top as the norm. Both top-to-bottom and bottom-to-top are traditions with historical precedent. A quick glance at my mostly more modern home library confirms that almost all have top-to-bottom orientated titles on the spines, with but very few with transverse or bottom-to-top titles. The same holds true for public libraries. And if you have wandered along looking at titles with your head tilted to the right, the ones orientated differently certainly stand out - if somewhat annoyingly! So with these three Idriess books. But why these three in particular? I am not sure. It may just be the whim of A and R (see, for example, A and R's rather idiosyncratic approach to editions in this site - https://www.idriess.info/single-post/2016/03/11/identifying-first-edition-idriess-books - or Editions, Impressions and Printings (Sep. 22, 2020) in this Forum). Any ideas?
There is one more thing to note with top-to-bottom titles: if they are placed flat with the front cover up the spine is readily readable. But with a bottom-to-top book, the title appears upside down!
PS, the first edition (1941) of Fortunes in Minerals has the title transversing the spine (DJ and hardcover spines); the second edition (1947) has the title from bottom to top (DJ and hard cover spines); yet the 1951 and subsequent editions have top to bottom titles. Make of that what you will!