This list is far from comprehensive and online research will reveal much more

CLUBS and Associations:
There are so many Austin 7 clubs that, in 1969, an association of them was formed - and this is now the go-to place that hosts a mass of essential data: https://a7ca.org It's a large site with lists of clubs, a superb archive of historical data, reproduction handbooks, chassis and car registers, A7 history, A7 model identification, events, etc. 

In the opinion of the writer, the best Austin Seven forum is Austin Seven Friends. This has the huge advantage of a searchable database of facts, experiences, and advice and is an invaluable resource that's added to most days.

Direct links to handbooks, parts manuals and sales catalogues
A comprehensive collection covering all years can be downloaded as PDFs: https://archive.a7ca.org/collections/handbooks-and-parts-lists/
A lovely collection of sales brochures can be found here: https://archive.a7ca.org/collections/show-brochures/
The Pre-War A7 club has a useful Facebook page: https://www.facebook.com/groups/pwa7c .... with other enthusiasts forming groups for specific models or regional interests.
As a club member, you will receive not only the club's magazine but also the Association Magazine, published quarterly since 1970 https://a7ca.org/about-us/magazine/

Background History and Books:
To read the fascinating background to the car, with reproductions of the notebook by Stanley Edge, the design draftsman, see: https://a7ca.org/austin-seven/the-people/
Austin 7 books are available from: http://www.pwa7c.co.uk/bookshop.php. This is an excellent collection of material with historical and technical literature and data on specific models such as the Type 65 and Nippy, Grasshopper and Ulster, etc.

Is your Austin 7 on the Car Register?
The A7CA provides a list of known vehicles at https://a7ca.org/chassis-register/  Check to see if your car is listed (or to add it, that would be very useful). It includes English Austin, American Austin and Bantam, German Dixi and BMW, and French Rosengart versions. You can search by Model Type, chassis, car, and registration numbers.
For the 100th anniversary of the Seven's introduction in 2022, a thousand cars attended a special event: watch the film here: https://a7centenary.com/
Chassis Numbers:
Most Sevens before the Ruby had the chassis number stamped into the nearside chassis rail just behind the rear engine bolt. This was accompanied by two narrow, aluminium plates riveted to the outside face of the nearside bulkhead; one stamped with the chassis number, the other the Car Number.
In the UK, many Sevens were registered with their log book using the Car Number, not the Chassis Number (the Austin Motor Co. always asked for the Car Number, not the chassis number).
Stamped into the transmission tunnel just behind the handbrake (the rectangular cover plate for the latter might need removing to find it) is the 'Model Type,', e.g. AF, AE, RN, etc.
By 1936, most Sevens used a Car Number incorporating the chassis number; e.g. for the Nippy, the plate had the Car Number prefix 'AEB' plus the body number - the latter stamped into the top of the transmission tunnel just aft of the handbrake.

Restoration, Mechanical Work and Spare Parts
For engine, other mechanical work and restorations, one highly recommended business with personal attention, vast experience of the Seven and much helpful advice is: http://www.albaaustins.co.uk Phone: 0141-942-8037

For new Austin 7 parts, try these links - in no particular order of recommendation:
David Cochraine at: https://www.a7c.co.uk/aboutus.php
Jamie Rogerson at: https://www.theaustinsevenworkshop.com/
William McKenzie at: http://austinrepro.com/ This company offers standard and also beautifully made, difficult-to-find reproductions of headlamps, speedometers, rev counters and various items for standard cars and also the Swallow, Nippy, Ulster, etc...
For both new, used, and some refurbished parts try:
Tony Betts at http://www.7ca.co.uk/ and http://www.southernsevens.co.uk
Ian Tillman at http://www.oxfordshiresevens.co.uk/welcome-to-oxfordshire-sevens/
A helpful link (though some parts are out of date) with details of other repairers, restorers, gearbox, suspension and bodywork experts, etc: https://oldcarservices.co.uk/austin-seven-specialists/ (some of these will be out of data)

Trimming:
Simon Laxton: https://www.7trimming.co.uk/
David Nightingale: https://www.coachtrimming.co.uk/

Technical Help and Data
The Cornwall Austin 7 Club's website is especially useful with lists of suppliers and much really helpful technical advice, etc. https://www.austin7.org/
If you want to know about 'Technical Data & Specifications', thread sizes, etc, this page will help: https://www.austin7.org/Data%20Pages/Tech%20Data/
Various useful technical articles: https://www.austin7club.org/tech_contents.htm
For detailed information about the Austin 65 and Nippy Sports models, see: https://aus7in.wordpress.com/
For detailed information about the Ulster Sports models, see: https://ulster7.wordpress.com/
For detailed information about Gordon England versions, see: https://gordonenglandregister.wordpress.com/

Interesting Odds and Ends
For 25+ pages of digitally restored, attractive and high-resolution Austin illustrations (some of which you can click on and download at even higher resolution) see http://www.lathes.co.uk/austin7

Fitting indicators
an essential guide is available here: http://www.lathes.co.uk/austin-7-indicators