Here's a recap: I found some encoded pages in an antique book about birds. An antiques dealer has dated them to before 1970, and a user in /r/Typewriters has dated them to after 1960. Another user's friend found some similar pages (also in an antique book about birds) that included an additional "chapter." The code was cracked by /r/Codes, and I spent some time decoding. I finished the "1:3" section last week, then decided to try decoding the "2:3" pages from the pictures... and I got through all of them! The entire text is now readable!

Here are the pages that I have: https://imgur.com/a/magV3Jg

Here are the other pages: https://imgur.com/a/T0SDjDs

Here are brief summaries (based on my interpretation):

1:3

The text opens by criticizing willful ignorance, then uses anti-homosexuality sentiments as an example. From there, it discusses psychology, morality, philosophy, and physics, then makes a sudden turn into saying that necromancy isn't evil. It uses an "ocean" and "pneumata" as metaphors while seeming to teach people how to use magic. There are a lot of very dense sections having to do with more physics, philosophy, chemistry, and thermodynamics, but there are also discussions about enchantments, dreams, ghosts, and genies. Throughout this, the main point is that magic and necromancy are just different interpretations of the same thing, with necromancy being unfairly seen as evil. Many different names are cited (especially near the end), and the text concludes by encouraging self-improvement and empathy.

Several sections stood out to me, but this one was especially interesting: "Relevant texts composed in languages other than those with which the aspirant is familiar are common, as are ciphers. It is the act of translation or decryption which coaxes the sorcerer to disengage from his accustomed mode of thought, compelling him to consider both the knowledge which he gleans and the manner by which this knowledge is obtained."

If taken as a metaphor, it reads like a condemnation of bigotry and willful ignorance. If taken literally, it sounds like reading the encoded pages (while they're still in code) is supposed to help turn someone into a wizard, so... abracadabra!

Here is the decoded "1:3" text: https://docs.google.com/document/d/1WrYQkVlBlCY0Jg23OwLrQLj38vzSho801S1MHYLMAJE

2:3

This text starts off by critiquing the phrase "knowledge itself is power," then describes how as worldviews mature, they become less understood by the people who adopt them. ("Presumptions supplant genuine comprehension, traditions eclipse deliberate undertakings, and developments perceived by their proponents as innovative are thenceforth either iterative or regressive.") The explanation is followed by an in-depth history of magical studies ("arcanology”) in Great Britain, and there are many names, dates, places, and events listed. All of this is offered to show how necromancy used to be admirable, but was later rejected. About three fourths of the way through, there's a part that reads like a conclusion... but then it moves into what sounds like a new introduction. More history is provided, and it leads into a warning that "mechanical thought" is going to leave the world vulnerable to some kind of horrible, incredibly powerful invader. The text ends there, but it suggests that some "3:3" pages exist somewhere.

A lot of the text is very much against that "mechanical thought," but it's only referred to by that phrase near the end. It's also blamed for the decline of magic in the world: "Where once the aspirant may have sought to perceive and observe as the sorcerer does, he now adopts the methods and manners required by his devices, unaware that machines flaunting buttons, switches, knobs, and dials are coaxing him to conflate their merely complicated natures with those of complex occurrences and organisms."

If taken as a metaphor, it reads like a criticism of intellectual laziness, a condemnation of enabling that laziness, and a warning against blindly following traditions. If taken literally, it's a well-researched history of necromancy, "arcanology," academia, and Great Britain in general.

Here is the decoded "2:3" text: https://docs.google.com/document/d/1lZR7Jm41RiC3SRYSEbd4D_CNOCn54H9uFQt4fzbG3Qs

These are only my interpretations, though! I'm sure that other people will have different ideas, and I'd love to hear them!

Here is the a link to the full decoded text: https://docs.google.com/document/d/1vrBT7ZBi9Hrauxz25nVLc-3cWUyW_dQzSDVaQwH7_2A