Asm Guru is a series of tutorials written by James Matthews, aimed at teaching Z80 Assembly for the TI-83 calculator. It was started in 1998, with the last update being published in January 1999.
The tutorials were unique in that they were written by the author as he learnt each topic himself. Because of this, the order of the tutorials and topics covered doesn't necessarily follow the "traditional" order of taught programming subject matter. However, many topics are covered that give readers an insight into the meaning of the Z80 instructions and the available ROM calls on the TI-83.
The file also contains an extensive reference section, with documentation of the Z80 instruction set, including an OPCODE table and also summaries of how certain instructions affect the flag register, etc., as well as tutorials by other authors.
After the TI-83+ was released, another tutorial, Ion Guru, was released (by a new author, Matt Hernandez), which dealt with taking the subject matter from ASM Guru and applying it to Ion, for compatibility between the TI-83 & TI-83+ calculators.