To learn electronics (which is pretty much essential), then a copy of The Art of Electronics, by Horowitz and Hill (ISBN: 978-0-521-37095-0), is indispensable.
Nevertheless, on its own, it might not get you up and running as quickly as you like. Also, as you want to build a smart device, you will probably also need a controller of some sort, and you have mentioned the Pi (or Pi kit).
However, as an alternative to the Pi you might want to consider Arduino (or Arduino beginners kit).
Arduino is a lot simpler than Pi, with a lot less over head, code footprint wise. Yes, you can get a Python script running quickly on a Pi, that is true, so the Pi is simpler in that respect.
However, with Arduino, you are coding onto bare-metal, so to speak. You write and compile a "few" lines of code, and upload that to the Arduino, and that is all of the code that you need. Whereas, with Pi, you (usually) end up writing a Linux based application, to run on an OS. So, in that respect, Arduino code can run faster (generally - although some more argue otherwise), as there is no code running for screen updates or keyboard input, and all of the other baggage that comes with a full-blown PC (which is what a Pi is, albeit a rather small one).
Arduino comes with countless libraries so, when it comes to interfacing the Arduino with a particular device, most to the work will be done for you. There's also a lot of ready-to-go examples and projects out there for you to copy or adapt for your own purposes. So, while you will need to, or should, learn electronics (in the end), for a while you may be able to get away without learning/knowing the theory.
That said, I have a few friends (with no electronics training/background), who have tried Arduino, and after trying to run before learning to walk, they have sadly given up. This is because they refused to start off slowly with the excellent basic examples provided and tried to do big projects without any understanding of either C/C++/OOP and/or electronic theory.
You could counter all of the above, with equal arguments for the Pi... It really depends on what you want to do. For simple devices, like wearables, which you seem to refer to, then Arduino would seem to be more sensible, due to the possibility size reduction. For large complex projects, with a lot of data processing, a Pi would be more useful (as the memory (and processing power) on an Arduino is rather limited). Of course, you could use both, with an Arduino monitoring sensors and feeding that data back to the Pi for processing and display on a large monitor.
Depending upon your requirements for quantity of IO ports, you can make large or tiny devices (using large (Arduino Mega 2560) or tiny (Arduino Micro) Arduino based boards respectively), or just the µController IC itself with a few extra support components (two capacitors and a crystal), and the the components for your project (i.e. LEDs, speakers, sensors, or what-have-you).
Finally, Arduinos can be a lot cheaper than Pi - if you go for the Chinese clones, they can be found for a few dollars, and work just as well, in my experience.
Either way, in addition to the very good Arduino and Pi support sites, both Pi-SE and Arduino-SE are excellent Stack Exchange sites, right here, as well.
If you really need faster devices then look into FPGA, but that would seem to not be necessary for you yet, nor is it really for beginners.
