A slow thread, the best modern book on my areas of study are, British Hoverflies by Alan Subbs and Steven falk (2002) and British Soldierflies and their allies, 2nd editidion (2014) by Alan Stubbs and Martin Drake and the Field guide to the Moths of Great Britain and Ireland, 3rd edition (2017) by Paul Waring and Martin Townsend and Richard Lewington is indispensable.
All the interest in these butterflies, all those specimens in big collections, many experts (I am not one of those I am afraid) so where is that up to date book on genus Delias, I and you might ask? The last book on Delias was published in 1993, text in Japanese, only readable by those in Japan, written a quarter a century ago, now greatly out of date, surely this is one of the greatest omissions in the study of Lepidoptera!!
I agree that a more up-to-date treatment of Delias would be nice to have. The Japanese book has a number of errors, in addition to being considerably outdated (and Japanese-only text, of course). There's a project in the works that may be done as a website rather than in printed form, which will enable a far larger number of illustrations. I've not heard any news about it in awhile; I'll have to look into it again.
Web sites are great but still need to be updated. You cannot beat a good book during the winter months in a cosy arm chair. I guess there are so many species of Delias and innumerable subspecies (much too many in my opinion, almost every locality in New Guinea has a different one) You would need Delias of the World vol 1 and 2. One might expect perhaps the Japanese to do this, but this time published it in English as well and you will sell far more. I have seen some amazing photographic plates of most of the species and subspecies, its a pity they are not heading for a book. I guess the disadvantage of a book, is that as soon as one is published, a new species/subspecies might be discovered, but these are generally not an every day occurrence, and this will happen with most book on butterflies. Sometimes I wonder If the work by so many different taxonomists, makes a mountain out of a molehill!! Then presenting an almost insurmountable headache for authors contemplating such a work as Delias?
I guess if a book is still useful that is the most relevant thing. All books historical or modern can be indispensable. I have now changed the title to this thread "Your most useful Entomological books" and what would you like to see published in the future.
There are so many useful books on Papilionidae, both old and new. My personal most useful books at any one time really depend on what I am studying, but in general I guess I would have to highlight the numerous Japanese publications such as Tsukada & Nishiyama, 1982 (Butterflies of the SE Asian Islands, Papilionidae, English edition); Matsuka, 2001 (Natural History of Birdwing Butterflies); several books by Igarashi, but particularly his 1979 Papilionidae & their Early Stages; Fujioka et al., 1997 (Japanese Butterflies and their relatives).
One of the most useful books that I use on a very regular basis is “The Butterflies of West Africa” by Torben B. Larsen. This was printed in 2005. The text for each species and subspecies is first class and the fact that the plates are in a separate volume to the text is extremely useful as the text and plates can then be viewed at the same time. It would take a lot of very hard work and research to improve this work as it is already quite comprehensive..
In the UK we are very lucky in that we have specialized field guides on most insects orders, a great omission is of the Coleoptera. The last field guide was the two volume set by E. F. Linssen in the 50s and now vastly out of date. Of course you could not include all the British species in the field guide but you could include those you are most likely to find to at least genus level. If the book was in the same vein as Bloomesbury Wwildlife Guides, their books on moths being a good example, you could fit most of the species that are found in Britain in it, with a good key to those species.