Modular Synthesis, How To
For those looking for the Synth Secrets PDF, all 63 of the original articles are available on the Sound On Sound site via this search results page:
Quite honestly there's little else out there as free resources that are better at teaching the fundamentals of synthesis - modular or otherwise. It's certainly no idiots guide and it can be a little wordy in places but it really does cover all the bases. The only area where it's comparatively weak is 'east coast' synthesis methods. But IMO Buchla/Serge synthesis methods are easier to learn if you already have a good working knowledge of subtractive and FM. Especially with a modern-day 'zero coast' mindset where you're creating complex source material via FM and waveshaping and then mixing it up with subtractive techniques.
Another great resource is Chris Meyers series of Lynda.com videos. Forget the Arturia Modular ones, but both the Moog Mother-32 and it's Eurorack 'Learning Modular Synthesis' follow-on are excellent. These obviously aren't free but they're worth paying for.