This is a mine field, I'm sorry to say.

Most of her biographers approach her with an agenda. There is a lot of attack literature from the political right against her. This literature tends to emphasize her eugenicist beliefs and her association with unsavory types in the eugenics movement, in addition to making things up whole cloth. The political left, on the other hand, tends to lionize her, but if you can get past the hero worship, you'll probably get a slightly more accurate view of her.

That being said, she was against abortion, and authors on both sides of the political spectrum tend to minimize or deny this. This wasn't some tangential part of her thinking, but one of her major motives for birth control advocacy. For these reasons, I tend to be wary of secondary sources about her.

To answer your first question, I'd recommend starting with her autobiography for a couple of reasons and with a couple of caveats. By starting with her as a primary source for her own life, you won't get your view biased by the filter of modern political agendas. Her autobiography is in the public domain and free. Of all her writings, it tends to stand up the most. On the downside, her autobiography is on the longer side (>500 pages) and there are whole chapters that are basically her travel blog. (She was really, really into traveling.) It is also affected by the self-aggrandizing tendencies all autobiographies have.

To answer your second question about whether her writing stands up on her own, in a word, no. Having read a couple of her books and many of her speeches and articles, I've noticed her books aren't really coherent manuscripts as they are collections of her speeches and articles rewritten into a book. This makes them difficult for a couple reasons. For one, they have a rhetorical style you would expect from someone making a speech in the early twentieth century. This is why I say her autobiography stands up the most of all her writings: it is written in a style like someone explaining her life to a friend, not like someone making a speech you are listening to.

Furthermore, a lot of the topics she addresses in her writings are derived from other movements and ideas that she allied with. For instance, there is a lot of neo-Malthusianism and negative eugenics sprinkled in her writings. These are largely extinct movements, and I suspect context for them would help in understanding of writings.

In summary, I'd suggest starting with her autobiography. With the exception of her autobiography, her writing generally doesn't stand up too well on its own and is better read alongside something to give context. But most works that purport to give you context will be filtered through the lens of modern political agendas that are incorrect to apply to her ex post facto.

Reading her is perhaps not as challenging as my answer would make it seem. I just wanted to be sure to answer your questions. She really was an interesting person, and I don't want to discourage anyone from reading about her.