I can’t give away too many details as it could be identifying but I do have the following in my small collection:

• A mechanics textbook from 1890 ish that has really good colour illustrations of steam engines and cotton machines.
• An air force booklet for trainees in the 1940s to teach basic maths and physics starting from simple addition and subtraction written out in words.
• My most recent addition: a history of electricity and magnetism covering anthropological records dating back to the early days of civilization up to Faraday’s experiments which the author dedicated to Lord Kelvin.