List of recommended books:
www.designers-guide.org/Books/

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Forums (alphabetical order):
http://electronics.stackexchange.com/
http://forum.allaboutcircuits.com/
http://www.designers-guide.org/index.html
http://www.edaboard.com/forum.php
https://www.circuitlab.com/questions/
https://community.cadence.com/cadence_technology_forums/f (only for Cadence’s clients)
https://www.researchgate.net
https://www.semiwiki.com/forum/forum.php

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Facebook’s groups:
Analog RF Mixed-Signal IC Design & Development Group

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LinkedIn’s groups (alphabetical order):
Analog Mixed-Signal and RF (AMS/RF) IC Design and Development Group
Analog SemiConductor | Europe
RF, Analog and Mixed-Signal IC Design Engineering
RF/Analog/Mixed signal circuit design & product/test engineering
VLSI