For Breton names:
The Celtic Baby Names Book by Gillian Delaforce
Baby Name Encyclopedia: The Perfect Baby Name Adviser by Sylvie Nicole
Baby Names That Go Together: from Lily, Rose, and Violet to Finn and Fay - sibling names that mix and match in a perfect way by Eric Groves Sr.
Brilliant Baby Names by Laura King and Geoff King
Simply the Best Baby Name Book by Stuart Wilson
For Cornish names:
The Celtic Baby Names Book by Gillian Delaforce
The Virgin Book of Baby Names by Emily Wood
Baby Names Made Easy: The Complete Reverse-Dictionary of Baby Names by Amanda Elizabeth Barden
A World of Baby Names by Teresa Norman
The Penguin Book of Baby Names by David Pickering
Brilliant Baby Names by Laura King and Geoff King
The Complete Book of Baby Names by Lesley Bolton
Baby Names Made Easy: The Complete Reverse-Dictionary of Baby Names by Amanda Elizabeth Barden
Baby Names That Go Together: from Lily, Rose, and Violet to Finn and Fay - sibling names that mix and match in a perfect way by Eric Groves Sr.
Best Baby Names for 2017: Over 8,000 names and 100 inspiration lists by Siobhan Thomas
Simply the Best Baby Name Book by Stuart Wilson
For Irish names:
The Celtic Baby Names Book by Gillian Delaforce
A–Z of Irish Names for Children and Their Meanings, Finding the Perfect Irish Name for Your New Baby by Diarmaid Ó Muirithe
A Genealogist's Guide to Irish Names, A Reference for First Names from Ireland by Connie Ellefson
A World of Baby Names by Teresa Norman
Brilliant Baby Names by Laura King and Geoff King
Simply the Best Baby Name Book by Stuart Wilson
O'Baby by Geoffrey Johnson
Baby Name Encyclopedia: The Perfect Baby Name Adviser by Sylvie Nicole
Baby Names Made Easy: The Complete Reverse-Dictionary of Baby Names by Amanda Elizabeth Barden
For Manx names:
The Celtic Baby Names Book by Gillian Delaforce
Simply the Best Baby Name Book by Stuart Wilson
For Scottish names:
The Celtic Baby Names Book by Gillian Delaforce
A Genealogist's Guide to Gaelic Names, A Reference for First Names from Scotland by Connie Ellefson
Brilliant Baby Names by Laura King and Geoff King
Celtic Baby Names: Traditional Names from Ireland, Scotland, Wales, Brittany, Cornwall & the Isle of Man by Judy Sierra
Simply the Best Baby Name Book by Stuart Wilson
For Welsh names:
A Book Of Welsh Names by Trefor Rendall Davies
The Celtic Baby Names Book by Gillian Delaforce
A World of Baby Names by Teresa Norman
The One-In-A-Million Baby Name Book: The Babynames.com Guide to Choosing the Best Name for Your New Arrival by Jennifer Moss
Naming Your Baby: The definitive dictionary of first names by Julia Cresswell
Simply the Best Baby Name Book by Stuart Wilson
Celtic Baby Names: Traditional Names from Ireland, Scotland, Wales, Brittany, Cornwall & the Isle of Man by Judy Sierra
The Penguin Book of Baby Names by David Pickering
Personal knowledge of Welsh name pronunciations