Quickly going through my own collection, the following White Dwarf magazines are most useful to the Tau:
- White Dwarf #261, #262 and #263 (US). Perhaps the most quintessential issues of White Dwarf for the Tau, these three issues were the first ones to cover the original Tau release in 2001. #261 features an Index Xenos background overview of the Tau Empire, as well as some Tau concept art and short stories.
#262 features the original designer's notes on the Tau, more concept art, articles on the design process behind the original 3rd edition Tau plastic kits (which were the foundations of the Tau design aesthetic), an Index Xenos background overview of the Kroot (which includes material that I don't think has ever been published elsewhere), a Tau miniatures gallery, a showcase of a Tau-themed cityfight table, a collection of Tau related short stories and the first ever battle report to feature the Tau (with none other than notorious enemy of The Empire Phil Kelly commanding the opposing force).
#263 features an article on Crisis Suit configurations (which is where many of the Crisis Suit loadout naming conventions originate from), a background overview of Vior'la (which I don't think has ever been published anywhere else) and a tutorial on how to make a Tau building scenery piece.
- White Dwarf #307 (US). While most of this issue is focused around coverage of the 6th edition Wood Elf and 4th edition Tyranid releases, it does also feature a Golden Daemon showcase that includes some Tau models.
- White Dwarf #313 (AU). Not only is this the first White Dwarf issue I ever owned, it is also the first White Dwarf to feature the Tau army of Tael himself, which is showcased in an article that includes detailed commentary from Tael alongside selected model close-ups and a couple of quick modeling tutorials (like how to make a separate or discarded Firewarrior helmet). Also featured is a preview for the 4th edition Tau codex.
- While I do not own a copy of it (yet...), and thus cannot provide an issue number, if memory serves the issue for April 2006 was the one to cover the 4th edition Tau release, and includes a battle report between the Tau and Dark Eldar amongst other Tau-related articles.
- White Dwarf #316 (AU). This issue features an introduction to the Medusa V campaign (including a teaser for the Tau side of it), an Index Xenos overview of the Vespid, a showcase article for Troy Forster's Tau army and a modelling and painting guide for Tau vehicles (including one of the first guides to Sept colours).
- I can't seem to find an issue number on it, but April 2013 is the White Dwarf issue covering the 6th edition Tau release. It features designers' commentary on the 6th edition Tau models, a battle report between the Tau and Tyranids (odd how history repeats itself sometimes) and a Tau painting guide.
There's almost certainly more (there was a second White Dwarf to feature Tael's army, this time focusing on his cityfight force, and I can dimly recall a battle report between Tau and Dark Angels, but I can't for the life of me remember what issues those articles were in), but those are the ones I have close at hand and should provide a start.