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.