Lewis Allen's Printing With the Handpress may be the most widely known of 20th century books on the subject. He, along with his wife Dorothy, spent 50 years designing and printing books that took the fullest possible advantage of the techniques and freedoms offered by the handpress, such as printing multiple colors with a single impression and using wonderful handmade papers (which, of course, had to be printed damp to realize their fullest potential). By the mid-1960s Lewis Allen had become recognized as one of the few practising experts on handpress printing, and he spent many hours replying by mail to inquiries about technique, materials, etc. To avoid answering the same questions repeatedly, he and his wife decided to print a manual describing the equipment, materials and techniques used at the Allen Press to publish a book.
The book is illustrated with many drawings (line engravings) by Victor Seward, plus some decorations by Mallette Dean. It is narrower in scope and shallower in depth than Rummonds' book, but these features may make it a less daunting introduction to the craft. It covers all the same bases, but in less detail.
As a manual, Allen's book has never been used very much at HM. His method for dampening paper is more laborious and less reliable than Rummonds'; his mention of adding formaldehyde to the water has led subsequent generations to live in fear of carcinogens and mould (it's really not an issue); the humidors he describes for keeping paper damp are baroque. The book's real value lies in its production, using all of the techniques and materials described: you can examine each page to see the results of what Allen is describing. Which is why the subsequent facsimile editions are poor seconds. They contain the information only. This is the problem with all books about printing techniques that do not employ the actual technique(s) being discussed: you are left looking at a reproduction of the thing, which is never the same.
Nonetheless, the original edition of Allen's book (limited to 140 copies, the usual run for Allen Press titles) is not often encountered now, and never inexpensively, so most people have to make do with a copy of the facsimile. The drawings are excellent references, the book is beautifully designed and printed. Its concise description of the process for designing, setting and printing a book is an excellent primer for anyone wanting to develop a knowledgeable appreciation of fine printing. Despite our niggling disagreements over technique, HM holds the Allens' work in high regard - aspirational even - and will always credit them with opening our eyes to the potential of the handpress.