does Barlow's work really count as canon at all?
I'm not Ningauble, but I'll take a swing at this.... No, Barlow's pieces aren't what most would consider "canon"; they are revisions of the work of another and, unlike some of the revisions he did for others, he didn't introduce elements of his "mythos" into any of them... though there are elements of the cosmic perspective so much a part of Lovecraft, especially in "The Night Ocean"... though that would seem to be Barlow's contribution as much as (if not more than) Lovecraft's, in that particular case.
Because in the mean time, until a complete collection comes along, I'm only interested in essential stories that'd allow me to understand this "mythos" of his.
Understanding his "mythos" is... a thorny question, as there are differing opinions as to what constitutes the mythos. If you're interested in Lovecraft himself, rather than the (so-called) Cthulhu Mythos, I'd suggest reading just his work first, before dipping into that of his colleagues and all the other contributors (some of which are definitely worth looking over, though there's a lot of absolutely dreadful stuff under that rubric, too....). That way, you have a much better idea of what Lovecraft himself was attempting to, and indeed did often, accomplish.
Furthermore, could you shed some more light onto the B&N volume and explain exactly how crucial are these revisions for newcomers like me?
The revisions in general -- assuming you are referring to those tales he revised for others, rather than the corrections to be included in the second printing of the B&N volume -- are less important; a lot of what HPL added to them was as much "window dressing", in many cases, as substantive. There are a few exceptions, however: "The Mound", "The Curse of Yig", "Medusa's Coil", the stories he revised for Hazel Heald, and the like, do add contributions to the conception of the mythos, though they are sometimes at odds with "canonical" passages ... something which Lovecraft himself, it would seem, had in mind, knowing that genuine mythology doesn't really have a true authoritative "original", but instead numerous variants throughout different cultures and times.
As for the corrections themselves... they can be vital, as they can entirely change the reading of various passages in Lovecraft's work, and cumulatively, this can alter the reading of the piece as a whole (
At the Mountains of Madness is a good example). For all their faults with the new typos and such, as you're looking at a budget here as well, I'd suggest going for the 3-volume Penguin edition of Lovecraft. They are relatively inexpensive, much, much better than the del Rey editions, are much closer to an authoritative text (with one or two notable exceptions), and lack the editorial mucking-about which are a hallmark of so many of the older texts unfortunately still used for the majority of cheaper editions of his work. They also include the text of "The Shadow Out of Time" taken from Lovecraft's original ms., as opposed to the badly-edited version published in Astounding Stories, as well as the revised "Hypnos", and including the attribution (which provides the name of the narrator) for "The Call of Cthulhu". In addition, they are chock-full of informative annotations by S. T. Joshi as well, which can add to the varying layers of understanding when it comes to HPL's often dense work.
These volumes are:
The Call of Cthulhu and Other Weird Tales
The Thing on the Doorstep and Other Weird Tales
The Dreams in the Witch House and Other Weird Tales
And, of course, if you're interested in the revisions (
The Horror in the Museum and Other Revisions), the del Rey edition's text of the tales is taken from the revised Arkham House edition....