8051 microcontrollers are old but stil very popular. There are number of books is written about them and almost every IC manufacturer is producing 8051 core products. And why not, because there are tons of very useful products made upon 8051 microcontrollers. You can find very efficient microcontrollers with 8051 architecture(Flash, EEPROM memories, multiple peripherals, ISP, you may find ones that executes one instruction per clock cycle).
Mahesh Wankhede have edited nice handy tutorial about 8051 microcontroller interfacing to peripherals like I/O Ports LED, Switch, Relay, ULN, RS232C Cell Phones, GPS receiver, 7-Seg. Display, LCD Display, ADC & DAC, Sensors, Stepper Motor, DTMF Decoder, Keyboard, External Memory with relevant theory, schematics and sample codes in Assembly language and C language. For someone this may be a kick start to a new project.