seek_shangrila is to be credited and thanked for discovering this. Reading it online isn't as good as having the actual book, but since Marie Larisch's books are extremely difficult to find at reasonable prices, an online version is better than nothing. This page also has a bit more text on her, Mayerling and her role in the T.S. Eliot "The Waste Land" (apparently she is the Marie mentioned in the first part, I hadn't realised that, but I also hadn't read that poem since I got interested in Elisabeth etc.), and includes some brief excerpts from her other book, The Secrets of a Royal House.
There shouldn't be anything illegal about this being online, as the work is well out of copyright (having been published in 1913) and also out of print.
A note to those who don't know: Marie Larisch was Elisabeth's niece, and used to be her close friend and confidante. But after the Crown Prince Rudolf and Mary Vetsera committed suicide at Mayerling and it turned out that Marie had introduced them to each other and acted as a go-between, facilitating the affair, the outcome was partly blamed on her and Elisabeth turned against her and she was shut out of the court. Her books are not considered to be the most credible sources - she was very embittered against Elisabeth and seems to have taken the opportunity to tell lots of gossip and lies, and it's uncertain how much of her memoirs can really be trusted.
But still, it can be an interesting document to those who are interested in Elisabeth, Rudolf etc. Marie Larisch's writings are frequently referred to in biographies of Elisabeth and Rudolf (even if often to say that she wasn't telling the truth) and I at least have become interested to see what she actually did write. And probably a part of it is trustworthy. I've read a few chapters, and at least so far her descriptions of Elisabeth's character and daily life don't seem that inaccurate, though I won't trust a word she says or implies about Elisabeth's supposed affairs, among other things. But someone who is a complete newbie to Sisi (or Rudolf, who Marie seems to be trying to discredit very hard) shouldn't start with this book, it's better to have read some credible biographies first so that you have an idea of what might possibly be trustworthy and what definitely isn't.