2024 is the 100th anniversary of the first crossword puzzle book, appropriately titled The Cross Word Puzzle Book. Back in 2022, the book became part of Project Gutenberg and so now lives royalty-free online. This page is a collection of digitized puzzles (with solutions) from this book transcribed into various formats convenient for solving.
Obviously, the quality of the puzzles themselves doesn't match up to modern standards, but they are quite remarkable given the lack of computational assistance available at the time. As one might expect, the density of crosswordese and archaic words is higher than what is seen in today's puzzles. And although symmetry rules haven't changed in the last century, other conventions such as no two-letter words, repeat answers, or unchecked squares (known back then as “unkeyed” squares) had not arrived yet. There is also an absence of wordplay, which is underscored by clues being referred to as definitions. While this makes some of the answers trivial (if you see the clue "Editor (abbr.)" for a two-letter word, don't overthink it), on balance, these factors make solving quite difficult. I would describe the experience to be more interesting than fun, although your mileage may vary. If you are looking to start in the shallow end of the pool, I would suggest puzzle 17, A Tesselated Tangle.
The book does not include an answer key. Solutions to some of the puzzles linked below were found in old newspaper archives from 1925. However, the majority were solved manually so there are no guarantees that they are correct. If you think you've spotted a mistake, please let me know at firstname.lastname@example.org.