While physical copies are now vintage collector's items, digital PDF versions are highly sought after for their portability and ease of searching. Many engineers keep a copy on their tablets or workstations as a quick reference for "building block" circuits.
This specific volume is part of Elektor’s famous "300 series" (like the 301, 302, and 303 predecessors), which compiled the best circuit snippets, "think-links," and DIY projects from their monthly magazines into one massive reference guide. Where to find the PDF elektor electronics 304 circuits pdf