The new tiered harvesting mechanic for Blood of Luclin, Shadow Harvesting, got off to a bit of a rocky start, but as of 9 January 2020, it seems to have stabilized enough for me to document it. Since I know some folks are prone to skimming, the summary (Too Long, Didn't Read) comes first, then a detailed explanation follows.
You can see shadowed nodes for 30 minutes after using either the Shadowed Staff of Luclin from the BoL Tradeskill Line OR from the Glass Orb of Swirling Shadows that can be occasionally (manually) harvested from a normal node. From the chat message when you harvest the orb, it seems that your chances for harvesting the orb are based on your shadow harvesting skill, so shadow harvesting mount gear has some uses.
The Glass Orb of Swirling Shadows is LORE, so watch for it dropping into your bags and use it as soon as possible so you can get another one!
Once you actually start to harvest a shadowed node, any shadowed node, the timer for seeing (and thus harvesting) shadowed nodes goes down to 30 seconds, so read the rest of the explanation before you "waste" charges.
Shadowed nodes have a chance to spawn when a normal node spawns. Shadowed nodes depop an hour after they spawn, so they don't get "stuck" in hard-to-reach spots. If a zone has been open for a long time without anyone harvesting normal nodes, then you may find no shadowed nodes anywhere you look.
Clearing a chunk of an overland zone of visible nodes, to force some respawn is a "Good Thing". Add in the chance to harvest an orb, and it is a "Very Good Thing" to do. Then add in the fact that shadowed nodes can spawn in the same exact spot as a normal node, then clearing normal nodes to prevent mistargeting during speed-shadow harvesting is a "Very Very ... " you get the idea. Let's just leave it at: "Clearcutting is good for several reasons."
Personal recommendation: While some folks love to travel with an entire stampede of fluff pets, pony, visible familiars, etc surrounding them, I strongly recommend putting away as much clutter as possible. The darn things just LOVE to get underfoot at the worst possible time, and can block you from seeing/harvesting things as quickly as you'd like. If you're harvesting in an aggro area, keeping a combat pet and/or merc handy might be good. (I recommend clearing aggro mobs in your chosen area as well, before you start shadow harvesting but sometimes you'll still be startled by respawn, and your pet/merc can distract them while you harvest.)
The new model is that ANY shadowed harvest can be used to make Expert spells. (Recipes can be found in Shadow Sage, Shadow Alchemist and Empyral Jeweler books, all of which are VERY rare from the longest Overseer quests, it seems.) Since some shadowed rares are also used for empyral gear, you may want to use mostly the more common shadowed bits (foods and the more common leather, wood, roots) for the experts, and save the rarer bits (below) for any gear you might want, including better shadow prospecting gear. But that will be your call, of course.
Leucosia of the French-language eq2artisans site provided this breakdown for us:
The two consumable-only classes, sage and alchemist, use Shadow ____ Studies books, all classes that can make gear or accessories get Empyral ____ Studies books. Yep, that's the only reason we can see for the inconsistency in book naming. :(