Immediately after I had bought the car, I also bought 2 books which have served me well and I highly recommend for any miata lover.
The first book is a must for any miata driver who wants to gets his hands dirty and wants to plan little projects - the Haynes Repair Manual. You can get it currently on promotion here, or from your local bookstore. I've personally referred to it countless times, usually just before I embark on a project.
Another book that I think is an absolute must for the Miata Lover is Norman Garrett's Mazda Miata Performance Handbook, which you can get from Amazon here. It's suffice to say that he was a former Mazda engineer on the Miata project. I learnt to appreciate the car I had bought many-fold because of what I read in this book. Another lesson I learnt from this book is an enduring one that lasted me though all of the years I've owned the miata - as much as I love modifying cars, I'd think many times over any modification for fear of screwing up a beautiful peace of work. I kept my Engine stock standard for most part of my ownership (5 years plus) with just Iridium plugs, Jackson Racing Ignition cables, a drop in K&N panel filter and the timing bumped up to 16-18 degrees. Even the stock airbox lasted me through a long time. One thing you quickly learn about the miata is that it is also quite easy to screw up the entire car. Anyhow, read the book, it'll make you fall in love all over again.
One of the great things about having a miata is that there are so many places to find great advice from. One of the advantages of being in Singapore is that I can go and post some queries on a US based forum, head to bed, and have a barrage of learned replies the next morning when I wake up. Of the forums I like, I found the Miata.net forum and the Miata Turbo Forum most helpful. If I'm not wrong, Bill Cardell occasionally still posts there. Your own local forum would also most likely be able to provide some help, especially with finding local parts. For Singaporean owners, here's the link again to the local club's garage section.
It's Garage section is superb and is a good place to start for most queries and generally a very good read. What I've found especially useful are
There are several vendors out there that provide a treasure trove of information and yes, many of them are run by true miata enthusiasts who aren't just out to make a quick buck or 2. Of these, I've found Flyin' Miata's tech section very useful, especially for the (closet?) geek in you. I've also found that their product support webpages are full of useful information, but you need to relate them to your own hardware if the items are different from what FM sells. Their ECU page also has some goodies and was most useful when I tuned my FM Link ECU. Most of the products also give you great ideas to think about.
Other vedors I check out once in awhile
There are also many other websites I've found useful and I try to list them as much as possible below.