This is my personal suggested reading list. Please note this list is entirely subjective and not a definitive, comprehensive list by any means.
X-Men #49-50
These two issues are Polaris' introductory issues from 1968. The image in my first post comes from #50. In these issues, we get a sense of Lorna as a young latent mutant whose powers are (re)awakened with a device by Mesmero. Fun fact: we learn Lorna was dyeing her hair brown to "avoid attracting the curious." These two issues are very important to understanding how Lorna entered into the Marvel/X-Men mythos and the foundation that all else about her grew out from.
New X-Men #132
I'm skipping decades of storylines (Malice, Zaladane, X-Factor) in making this suggestion. I feel everything else prior either treated her poorly or wasn't very interesting. Other people may feel otherwise, which is why it's worth it to check their suggestions.
In New X-Men #132, we see Lorna in the aftermath of an attack that decimated the mutant island nation of Genosha that was run by her father Magneto. She survived where millions of others did not, and she suffered in the wreckage hearing their voices ("recorded" with her powers) until the X-Men came and pulled her out. This whole story does an excellent job of showing the weight of what Lorna went through - and why it should never be ignored or forgotten.
Uncanny X-Men #425-426
I suggest these two issues not for importance or even necessarily good writing (there are some problems), but more for imagery and how threatening and dangerous Lorna can be. It's a good example of a story serving her point of view, and using her connection to her father Magneto to serve her development instead of the other way around.
Uncanny X-Men #431
Lorna shows Professor X (and Nurse Annie) the horror of what she went through during the mutant genocide attack on Genosha via exploration of her memories from that day. Works in conjunction with New X-Men #132.
Uncanny X-Men #442-443
Returning to the wreckage of Genosha, Lorna has a deep philosophical debate with Professor X over the values of pacifism, and whether or not use of potentially deadly force to stop anti-mutant hate is appropriate. Lorna does an excellent one-woman job of the struggle between Xavier and Magneto philosophies, further magnified by how they're standing in Genosha's ruins and Lorna is still dealing with the trauma of what happened there.
Civil War: House of M
House of M was an event within the 616 universe, but HoM is effectively an alternate universe. This miniseries chronicles Magneto's rise to power in that AU. Insofar as Lorna is concerned, we get to see her interact with her family, and we also get the very first depictions of both a child Lorna using her powers and its version of Lorna's mother Susanna. The version and relation of Susanna here would be the foundation for Suzanna in X-Factor #243.
Exiles volume #2
Another alternate universe depiction, but I highly recommend it. The team includes teen versions of Lorna and Wanda from separate AUs, and the story takes them to an AU built around House of M. It's very fun, upbeat and playful.
X-Factor #243
Lorna's origin story is finally told in this issue, over 40 years after she was created. This is another crucial issue to read for her as a result.
All-New X-Factor #1-2, #7-20
All-New X-Factor marked Lorna getting to lead a team in her own right, rather than "filling in" as leader for other characters. I say skip #3-6 because those issues really dropped the ball hard, but #7 and after mostly started to improve treatment of her. Important issue is All-New X-Factor #14, the first and only time Lorna got to have any interaction with Wanda in a decade. Just before the forced retcon on Wanda and Pietro's parentage led to Marvel keeping them apart.
Secret Wars: House of M
Another alternate universe. This four-issue miniseries does very well by its version Lorna, allowing her to be commanding, intelligent and resourceful. One of my fave highlights from it is banter between Lorna and Black Cat as Lorna picks a lock with her powers, and the idea of them criming it up together. However, I have to say that it did poorly by Magneto at one point and Quicksilver/Pietro throughout.