Unfortunately, many old versions (including the very early .txt and .doc files) are lost; here's all I could find:
These are stable links, which may be useful for referencing (at least if I decide to stay with this domain name, but I think so).

• 2017 03 24 (Current version)
• 2016 12 30
• 2015 07 25
• 2015 02 06b
• 2015 02 06a
• 2013 07 04 (First LaTeX version!)
• 2013 06 28
• 2013 05 18

Content-related changes

• 2017 03: Added more on Dirichlet-convolution-like operations and their properties
• 2016 12: Restructured into sections placed the (incomplete) section on diophantine approximation near the end of part I
• 2015 08: Added: proving irrationality of fast converging series (based on some Math.StackExchange questions and Fourier's proof of the irrationality of e)
• 2015 02: Added: magic squares
• 2014 08: Added: perfect numbers
• 2014 02-07: Added: Zsigmondy's theorem
• 2013 09-12: Added: cyclotomic polynomials
• 2013 09: Added: primitive roots

LaTeX changes (Some gaps filled in based on my TeX.StackExchange history)

• 2017 03: Fixed section numbering, improved overall referencing using autoref
• 2016 12: Finally fixed references (BibLaTeX+biber); Added index of symbols; fancy section headings using mdframed; made extensive use of titletoc (found out how to pass custom local information to titletoc using hyperref :) ); fancier font (libertine+newtxmath); circumvented label-overwriting through command overwriting inside labels
• 2015 02b: Started writing on low-budget gcd-calculus; Messed up page numbers in ToC
• 2015 02a: Automatized links to hints and answers
• 2014 07: Fancy boxes around theorems, lemma's etc (mdframed)
• 2013 07: First LaTeX version