 Public Domain Books to Restart Computer Technology
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CollapseOS is a project to make an operating system and tools that can help restart technology after a civilization collapse: https://collapseos.org/

Of course, the software is only part restarting computer technology.  The other part is building things like transistors or vacuum tubes and turning those into computers. 

If you only had one book to take back, I would recommend Understanding Digital Computers by Paul Siegel, which includes overviews of how transistors, vacuum tube, magnetic core memory and other things work. It shows how to make memory and logic gates from these, and how to put them together into a full computer. And more importantly the 1961 edition of this book does not seem to have the copyright renewed (checked at Stanford and LOC) so it is public domain and available at: https://archive.org/details/understanding_digital_computers

Of course, you might want more detail if you have to recreate computers, since manufacturing something with feature size smaller than a millimeter and control of the composition at better than parts per million. So I made a list of books that I believe are public domain (either because they were written by the government or because they are US books publish 1963 or before and the copyright was not renewed). This list includes ones with much more detail on transistors, vacuum tubes, magnetic core memory and also basic materials and science information that can be useful.


General Computer Information:

    Understanding Digital Computers https://archive.org/details/understanding_digital_computers
    Electronic Computers: A Made Simple Book https://archive.org/details/electronic_computers_made_simple
    Digital Computer Components and Circuits https://archive.org/details/digital_computer_components_and_circuits


Numbers with Computers:

    Arithmetic Operations in Digital Computers https://archive.org/details/arithmetic_operations_in_digital_computers
    Introduction to Numerical Analysis https://archive.org/details/introduction_to_numerical_analysis_hildebrand


Transistors:

    Transistor Circuit Analysis and Design https://archive.org/details/transistor_circuit_analysis_and_design
    TM11-690 https://radionerds.com/images/0/01/TM_11-690_BASIC_THEORY_AND_APPLICATIONS_OF_TRANSISTORS.pdf
    Transistor Technology Volumes 1 to 3:
    https://archive.org/details/transistor_technology_v1
    https://archive.org/details/transistor_technology_v2
    https://archive.org/details/transistor_technology_v3
    Semiconductors https://archive.org/details/semiconductors_hannay


Vacuum Tubes:

    Theory of Thermionic Vacuum Tubes https://archive.org/details/Theory_of_Thermionic_Vacuum_Tubes_E._Leon_Chaffee_Ph.D._1933
    TM11-662 https://archive.org/details/TM11-662


Alternative Computer Construction:

    Digital Applications of Magnetic Devices https://archive.org/details/digital_applications_of_magnetic_devices
    Relay Engineering https://archive.org/details/relay_engineering_1962


Basic Science:

    Principles of Chemistry https://archive.org/details/principles_of_chemistry_1963
    Elementary Quantum Mechanics https://archive.org/details/elementary_quantum_mechanics_fong
    Introduction to Theoretical Physics https://archive.org/details/introduction_to_theoretical_physics_wangsness


Materials:

    Metals Handbook 1948 https://archive.org/details/metals_handbook_1948
    Metals and Plastics https://archive.org/details/metals_and_plastics_1947

This list is unfortunately missing the technology that made computers cheap in the 1970s: mask produced integrated circuits using complementary metal oxide semiconductors (CMOS), since that was just starting to be developed in 1964 when copyright still applies. (Transistor Technology Volume II chapter 9 discusses field effect transistors and Transistor Technology Volume III chapter 5 discusses photo engraving which are precursors technologies.)

Never the less, having a paper copy of these books would help you greatly should you ever wish to restart computer technology after a civilization collapse.

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