If someone is interested in VHDL programming, there are two VHDL ebooks from Brock J. LaMeres published by Springer for free download as PDF and epub:
Quick Start Guide to VHDL (2019)
Introduction to Logic Circuits & Logic Design with VHDL (2nd ed. 2019)
Normally the first book costs about 50 EUR and the second one about 60 EUR as ebooks.
The free offer of ebooks should be a positive sign from Springer for free education during the Corona time and will be for free at least until the end of July 2020 (regarding to Springer).