The book by Ashenden  covers VHDL-93 in detail. Other books on VHDL include: Coelho ; Lipsett, Schaefer, and Ussery ; Armstrong ; Augustin et al. ; Perry ; Mazor and Langstraat ; three books by Bhasker [ 1992, 1995, 1996]; Armstrong and Gray ; Baker ; Navabi ; Ott and Wilderotter ; Airiau, Bergé, and Olive ; two books by Cohen [ 1995, 1997]; Pick ; Jerraya et al. ; Pellerin and Taylor ; Sjoholm and Lindh ; and Chang . Of these, the book by Armstrong and Gray and Perry’s books (two editions) are easy-to-read introductions. The following books describe example VHDL models for ASICs: Leung and Shanblatt , Skahill , Smith . Edited books by Bergé et al. [ 1992, 1993]; Harr and Stanculescu ; Mermet ; and Schoen et al.  contain papers on more advanced aspects of VHDL.
There are some issues and interpretations of VHDL that are covered in a separate IEEE document [IEEE 1076-1991]; also relevant are the IEEE standard logic system for VHDL [IEEE 1164-1993], the WAVES standard [IEEE 1029.1-1991], and the VITAL standard [IEEE 1076.4-1995]. The IEEE has produced a VHDL interactive tutorial on CD-ROM [IEEE 1164-1997]. Hanna et al.  cover the IEEE WAVES standard.
Updates and extensions to VHDL are controlled by the IEEE working groups (WG). These include study and WGs on: Object Oriented VHDL, Open Modeling Forum, Simulation Control Language ( SimCL), System Design & Description Language, VHDL Analog Extensions ( PAR 1076.1), VHDL Math Package ( PAR 1076.2), VHDL Synthesis Package ( PAR 1076.3), Utility ( PAR 1076.5), VHDL Shared Variables (PAR 1076; mod a), VHDL Analysis and Standards Group ( VASG ) Issues Screening and Analysis Committee (ISAC), VHDL Library, VHDL Parallel Simulation, and VHDL Test. Links to the activities of these groups, as well as an explanation of a Project Authorization Request (PAR) and the standards process, may be found at http://ieee.org and http://stdsbbs.ieee.org .