First of all: this isn't the first time this book has been proposed in this forum. Same story with one for disadvantages.
Anyways, while this always sounds like a good idea, there are many, many issues here.
- There has already been a supplement on wildcard skills.
- Techniques would also best be served as a separate supplement in the Power-Ups line.
- You know that Kromm has to be the one who writes this, right? The level of system mastery the book needs limits it to just one person, who has other tasks as well.
- Those other tasks are more likely to bring in money. What's the exact market for this? This book idea doesn't really expand the system much, as GURPS Powers did. It's more of what we already use for tasks we already have in-game.
- The various tasks could have their own supplements. Vehicle skills should be in a putative GURPS Vehicles, along with the catalog of vehicles. Animal and plant skills should be in a putative GURPS Bestiary, along with the catalog of animals. (This is a big reason I gainsay making these two supplements a series of PDFs. There can be supplemental series of PDFs for them, but there needs to be a main supplement for each one.)
- GURPS isn't a skill-based system any more than it's an advantage-based system. It's a task-based system. Want that jumping example? It's best off in a book focused on characters that need to jump often, like GURPS Skullduggery.