This guide is for the completist Engineer, who wants to learn every existing engineering recipe in the game. It has been designed primarily around Engineering skill level rather than character level.
There are times, though, where character level will intrude - where you must be a particular level if you wish to learn a given rank of Engineering skill, or where you must face foes of a particular level should you want to do some particular task.
This guide specifically omits recipes learned from trainers. If you cannot find Engineering Trainers, this guide is probably not of much value to you.
When the going gets tough, the tough go shoppin! Many recipes are available from vendors, in limited quantities or not. Part of the point of this article is to tell you where to go to get them...
Auction House trawling
Many recipes are world drops. That is, anything, anywhere, of the appropriate level can drop the recipe. Since there are many more tradeskills available than you can take, the odds that a given recipe drop being one that you can use is small. Your odds are much better looking to the auction house.
Hunting
As opposed to the above, some recipes are looted off of only a select number of creatures. For some, you need defeat only one (or a very few). Others are more elusive, and you may find yourself devoting days to the hunt. And some, you must defeat a boss in some instance to acquire.
Questing
Sometimes, the only way to learn a recipe is to do 'favors' for people. Or work the right lever on the right machine. Also, there are some quests that require the fruits of some tradeskill. While this article is primarily about gaining recipes, these quests may (or may not) provide opportunity to gain skill in your craft.
Reputation
Many recipes are privately held by a group, and you may only acquire it when you reach some level of favor with that group. As gaining reputation is more arduous (generally) than either gaining levels or gaining skill, these recipes have been listed separately.
Other
Some things just do not fall into neat categories. "Everything is miscellaneous."
Some schematics fall into several categories. Where possible, the easiest means of acquiring the schematic is the one mentioned.
Also, schematics that are Bind-on-Pickup(BoP) are noted as such. All schematics requiring reputation, and some vendor-sold ones, are Bind on Pickup.
Horde characters will find that going to any Matrix Punchograph 3005-B while they have a [Yellow Punch Card] or better will allow them access to the recipe for this item. Much easier than trying to beat it out of one of the alliance vendors....
Expert level engineering
Note that the quests for these items may require the character to be about level 35. Creatures between some of the vendors and "civilization" are between 35 and 40 as well.
Note: BoP recipes that drop off of specific mobs are typically visible only to engineers who do not already know the recipe taught. Recipes found during a raid are visible to the Master Looter.