For high-quality, legal digital copies that often include interactive boards, you can check these platforms:

: A 10-book curriculum designed to take a player from beginner to master. The final series includes Chess Evolution 3: Mastery , which is widely considered one of the most challenging and rewarding training manuals .

: Known for their "Move by Move" series, they also provide eBooks in PGN format , which is compatible with most chess software . Top Quality Chess Recommendations

: A tactical training system by Axel Smith and Hans Tikkanen that has become a staple for players looking to improve pattern recognition .

: They offer a curated "Essentials" collection that includes several Quality Chess series , such as the Artur Yusupov "Build Up Your Chess" books .

: This is a primary hub for digital chess books. They carry a large selection of Quality Chess eBooks and offer an interactive experience where you can play through variations directly in the app .