I'm not sure that is the same Gerould, but it's possible; he also wrote the introduction to the English translation of "The Danton Case" and "Thermidor" and has published several translations and a study of Witkiewicz. The book is vague on when precisely she was introduced (by her father) to morphine; it would have to have been circa 1921. From what I gather she did from thereon take it regularly until the end of her life. I didn't mean to say it didn't matter at all, only that considering all her surviving writing (save a few earlier letters) was probably written under the influence of morphine, it may not help to 'explain' the oddities of one specific work.
There are about ninety letters in the collection, the first from 1914, the last from November 1934. If you like I'll type up a list of precisely which they are - it would be interesting to see if the translators deliberately focused on letters on any topic in particular or omitted anything glaring. (There are also [...]s all over the letters, even the short ones.) Most are to her aunt (almost all that discuss the contents of her work in concrete terms - as opposed to chances of staging, progress or lack thereof - are), Helena Barlińska, a few interesting ones to Julia Borowa, some to Iwi Bennet, a few to Wacław Dziabaszewski, a few to writers (Bernanos, Cocteau, Mann). But the long diary-esque outpourings addressed at the aunt definitely make up the largest part. It says the selection is 'based on' a two-volume collection of her letters edited by Tomasz Lewandowski in 1978 and 1983.