Thank you! I thought the father of FIRE was Jacob Lund Fisker? ( https://www.amazon.com/dp/145360121X )
Unless I remember incorrectly, multiple income streams is a point made in https://www.amazon.com/Early-Retirement-Extreme-Philosophical-Independence/dp/145360121X I vaguely recall something in there mentioning combining income from a job + investments + paid hobby (maybe fixing bikes) + savings account interest... or something which was heavily lopsided haha.
As far as I am concerned, nearly everyone seeking early FI is working with multiple sources of income:
For other paid activities (really, gambling?... sigh....) I have a dual opinion on them. First off, if you have enough extra time, energy, and motivation to earn income through some other stream then sure you have more income AND it has the really important benefit that even after you quit your day job you can keep doing it if you enjoy it. On the other hand, I value my free time not quite as the following, but it gives you a general idea: http://www.mrmoneymustache.com/2012/10/18/why-your-time-is-worth-way-more-than-25-per-hour/ Now, I do not fully agree with a bit of what MMM writes about, as far as I can tell he is intentionally extreme just to make the point more memorable. However, the general point about time being valuable is something I do agree with. Sure, I could pick up a part-time job, I could learn to write novels and hope to be one of the precious few earning something noticeable, or plenty of other things. None of them are worth the time I would trade doing them I personally think. That being said, I will sometimes go beyond what is reasonable to sometimes cut costs, example walking a very stupidly long distance to-from work instead of just taking the bus, however that is done because it is also fun. A different example is hand washing clothes, absolutely not worth the time put into it, but the realization that I am doing something silly like that is amusing to me...
Not only is 40 hours / week of a minimum wage job totally livable, the author of this book would be able to save over 50% of such salary. Is it unusual? Yes. Is it easy? Haaaail no. But impossible? Please.
The result is a discipline that has transformed into managing uncertain outcomes in large heterogeneous models, i.e. complexity theory, rather than reducing everything to balls-and-sticks. Meadows was famous for devising "12 basic places to intervene in a system", nowadays the focus is on hedging bets adequately such that interventions don't catastrophically fuck up.
That said, some of the basic tooling is still flexible enough for basic business problems and some of the old gems are able to explain important concepts found in other fields without getting bogged down in the math.
https://www.amazon.com/Early-Retirement-Extreme-Philosophica... is my favourite, it's not about retirement, it's about using systems thinking to devise a robust lifestyle.
https://www.amazon.com/Introduction-General-Systems-Thinking... will make a good complement to Meadows and should give you a calculus to rigorously think of systems with.
https://www.amazon.com/Introduction-Cybernetics-W-Ross-Ashby... for its explanation on entropy, I mean requisite diversity, which will you give you an approximate mental quantity of how "powerful" any given system is.
https://www.amazon.com/Sciences-Artificial-Herbert-Simon/dp/... and https://www.amazon.com/Design-Rules-Vol-Power-Modularity/dp/... I haven't read either of these, but Herb Simon is extremely influential and has great thoughts on the notion of system hierarchies (nearly-decomposable systems is a great concept for design). The second book is about the properties of modular systems, which will help grok the reasoning behind a lot of refactoring techniques.
1] https://www.amazon.com/Early-Retirement-Extreme-Philosophica...2] http://www.mrmoneymustache.com