I would spend some time in the article and guides section. As for searching specific information I find using google then adding efour to the end of the query as my go to. “UK release efour”.
www.elitefourum.com/t/base-set-2000-a-guide-to-the-last-print-of-base-set/22312/1
www.elitefourum.com/t/wotc-chat-logs-from-feb-2000-to-nov-2000-q-a-w-employees/16872/1
www.elitefourum.com/t/identifying-shadowless-unlimited-base-set-boxes-packs/16993/1
www.elitefourum.com/t/shadowless-vs-first-edition-base/19077/1
www.elitefourum.com/t/base-pikachu-artwork-card-variations/15059/1
www.elitefourum.com/t/an-elaborate-attempt-at-print-run-estimation-wip-5-8-18/20273/1
Odds makes a good point. You don’t need to tell us every little detail of your book but provide a background of your knowledge or a table of contents or something so we are not shooting in the dark.
I assume if you are interviewing wotc employees you are serious about this.
Who is the intended audience? Serious collectors, casual collectors, or general pokemon fans?
What is considered early Pokemon cards? Base, Base to Fossil, Base to Rocket/Gym, Base to Neo, or all of wotc?
What do you mean by most comprehensive? All the cards to collect? There are a ton of errors in base set but it would just be easier to talk about high profile errors, or general error categories. Some errors were never corrected. Many errors are unique to a single card. Some errors are shared across many cards. There are also sample cards and prototypes.
Outside of the actual cards there are many things you can talk about too such as packs, decks, booster boxes, and uncut sheets. Then you have tcg-related items like counters, deck boxes, and coins.
What is your book concept? Is it similar to a catalog? Will I see a picture of every base card from Alakazam to the water energy? Or are you focusing around interesting informational tidbits and stories?
I highly recommend you look at the silver bible. While it’s a Japanese collectors book it is praised by collectors and fortunately for you there is a free pdf scan. It could give you some ideas when making your own book. And it’s not even the cards that make the silver bible great. It’s everything else that went into it.
www.elitefourum.com/t/japanese-document-archives-trainer-mags-silver-bible-pdfs/33002/1
Browsing efour might even dissuade you (in a good way) because of all the information already contained here. It’s easier to direct people here than write a book for them.