RS Components are an electronic part supplier who sell a large range of components and tools related to electronics. I have always had a good experience buying from them. They are also a stockist of Arduino boards and accessories, catering not only for the electronics industry but also for the electronics hobbyist and home user.
An op-amp is used as an oscillator in this LED flasher circuit. This is another circuit that can easily be built on an electronic breadboard by beginners in electronics. Click the above link for the circuit diagram.
Bebop to the Boolean Boogie 3rd Edition, by Clive Maxfield is a book that explains many concepts in digital electronics, starting with electrons and molecules, then how semiconductor components such as transistors and diodes work.
It shows how logic gates, which are made from transistors, operate and how other logic elements such as flip-flops and multiplexers are made from gates.
The author also explains IC design, manufacture and packaging techniques and then how ICs and other components are put together to make electronics systems.