One major (double) rule:
1° read read read !!!!
2° practice practice practice !!!!
A few more specific tips to learn more for free:
Read synth reviews all over the internet; you'll see that there are often different opinions about one and the same synth; learn their + and - , look at synth specifications to see evolutions and to see what are the bigger ones and the smaller.
Also learn about the universal basics of sound. And the basics of different sorts of synthesizing.
Read manuals !!!!! Not only the manual of a synth you own, but all manuals !!! I'm very serious when I tell you I've been reading the major part of synth manuals I found on .pdf on the web. Sometimes twice or more. I did even print out and read the manuals of most vintage top synth classics - Yamaha CS-80, nearly all vintage MOOG synths, ARP, ROland Jupiter-8 and Juno series, Korg MS-20, old modulars, etc... I hear you say "that's impossible; too much pages" - but I can tell you no, that's a wrong thought. Most classic manuals are very brief. The features are well explained and the basics of sound, each time in another way, but usually between *only* 20 and 50 pages (service manuals not included, but leave those latter, that's only electronical/technical stuff).
Read these before sleeping, in the bathroom, whenever you have some time.
Also there are a few very good books with very reliable info about (almost) every single analog synthesizer ever made; author is Peter Forrest - the title is "The A-Z of Analogue Synthesizers"and the book is in two parts: "A-M" and "N-Z". The books are in pocket formet but don't underestimate the source of knowledge they provide; this ís really an analog synth manufacturers & synth models encyclopaedia. No info about the universal basics of sound, but good descriptions about every synths, resumed features, some rumours, a very reliable quote for every synth, etc...
Other interesting books: "Vintage Synthesizers" by Mark Vail, "Analogue Days" may Trevor Pinch & Frank Trocco, etc...etc...
And as important: practice !!!! Don't expect to learn it all very fast; experience is the most important source of knowledge.
Do you have a friend who has one or more synths ? Ask to try them out. Do you have a music instruments dealer in your neighbourhood ? Ask him friendly to try out some synths or to see a demonstration.
And to finish I'd like to warn you: beware 'cause I can assure you the more I've been knowing about synths, the more synths I've had or tried, the bigger becomes that scary feeling that I don't know much yet...
