This is a somewhat useful guide, but it’s outdated, incomplete and often inaccurate. For example:

• Provides two table files. The first one has two lines that aren’t Thingy table format (*55 and 00/). The second one doesn’t work for the listed game, Pokemon Crystal — the values it provides for the letters are completely inaccurate.
• When talking about monster encounters, he states that they have a five-byte header “usually ending with XXYY191919″. XXYY is actually the map number, and the 19s are encounter ratios for the three times of day.
• The stats data he lists are incomplete.

If you know how to edit basic data structures, then you’ve basically outlived the usefulness of this guide. It doesn’t even start on pointers and how they’re used in Pokemon. All in all I give this a thumbs-up if you’re new to ROM hacking in general — it’s how I learned — but bear in mind that it’s got inaccuracies in several places, and is in serious need of updating.