Now tear your hair out. Because like most things Blender, the documentation is, quite frankly, nothing short of horrifying.
I have recorded my brief thoughts more as an aide-mémoire as opposed to an entire tutorial, or what this genuinely deserves which is a full-scale polemic.
Having actually have gone through the terrors and tedium of getting a simple job like loading objects from .blend file working, it seems prudent to record the resulting code and observations, if nothing than to prevent further harried, anguished and almost futile searches. Once the secret to this arcane incantation was finally revealed in all its glory, a quick series of experiments revealed the actual truth of the matter.
Far be it from me to criticise those wonderful and august mages who weave the spells that create the code that is the majestic and sublime which we all refer to in the vernacular as simply “Blender”, but when encountering the difficulties fraught in revealing the secrets of such a straight-forward task, one must stop and consider this altogether direct yet unsophisticated question: “What the Dickens were they thinking of?”
Now that my frustrations have been eased, here is – as we are wont to express oneselves in these harried times – the skinny:
bpy.ops.wm.link_append( filename = 'just_the_name_of_the_object_you_want_to_append', directory = '/the full/path to/your.blend/Object/')
Note that the spaces work just fine.
A caveat: the directory string must in in ‘/Object/’ with the trailing slash, if objects are what you are after.
Now, was that so hard? Not really, but when you look up the documentation you can see why it can make your eyes bleed.