Recommended reading for every magic player. This article explains some of the key aspects of sideboarding, with excellent attention to detail.
Thoughtseize You – by Ried Duke (Paywall)
A guide to (almost) everything you need to know about targeted discard spells. Recommended reading for anyone playing discard spells.
Who’s The Beatdown? – by Mike Flores
One of the most oft referenced pieces of magic writing, this article formed the foundation for understanding of role assignment for many players when it was first published on The Dojo. While the examples used may be a little tough to understand for newer players, the lessons it teaches are timeless.
Building Decks In The New Age: Combo – by Todd Anderson (Paywall)
Anderson takes an in-depth look at the type of combo decks in magic and explains their strengths and weaknesses He also takes a look at transitional sideboards in these decks.
Do As I Say, And As I Do Part 1 (Paywall)
And Part 2 – By Brad Nelson (Paywall)
Nelson takes a lot of shallow looks at topics such as challenging your assumptions before deck choice, how to test ahead of a tournament, why he plays midrange, how to use the information you have earned to make in game decisions and sideboard, and then some tips on bluffing.
Honestly, a little hard to read and get information from owing to its fractured nature, but worth the effort.