Printed
Britain’s Insects: A field guide to the insects of Great Britain and Ireland by Paul D Brock (Princeton Wild Guides 2021) [240-326 covers main families and some beetles]
A Field Guide in Colour to Beetles by K W Harde (English edition, ed by P M Hammond Octopus Books 1994/Blitz Editions 1998) [1]. Harde was one of three authors of the massive 10 volume book on beetles in German, Die Käfer Mitteleuropas. But this Field Guide is accessible and well-illustrated with colour drawings.
Practical Handbook of British Beetles by Norman Joy (Witherby 1932 / reprint Classey 1976, 1977) (2 volumes). Keys in volume one, with line drawings in volume 2. It was the definitive guide and is still useful.
Beetles of Britain and Ireland Andrew G Duff Three volumes of four published (AG Duff Publishing 2012, 2016, 2020). These are the definitive identification guides to British beetles for serious coleopterists.
Ground-Beetle Identification and Ecology Brian Eversham notes and key, 2008. The CEX of Wildlife Trust Bedfordshire, Cambridgeshire & Northamptonshire has given several talks at MKNHS meetings. He is keen for others to learn to identify beetles, so here is his introduction and keys to the 300+ British ground beetles: Ground-beetle identification and ecology
Ground Beetles by Trevor Forsythe (Naturalists Handbooks: 6; 2nd ed Pelagic 2000) [2]
A Key to the Adults of British Water Beetles Laurie Friday (Field Studies Council 1988)
The Larger Water Beetles of the British Isles Peter Sutton (Amateur Entomologists’ Society 2008)
Keys to the adults of Seed and Leaf Beetles of Britain and Ireland David Hubble (Field Studies Council 2012)
A Field Guide to the Ladybirds of Great Britain and Ireland Helen Roy, Peter Brown and Richard Lewington (Bloomsbury 2018)
Ladybirds by Helen E. Roy, Peter M.J. Brown, Richard F. Comont, Remy L. Poland & John J. Sloggett. (Naturalists’ Handbooks 10 Pelagic)
Ladybirds of the British Isles Majerus, Roy, Brown, Poland & Shields (FSC Fold-out chart 2006)
Ladybirds Larvae of the British Isles Brown, Roy, Poland & Shields (FSC WildID Fold-out chart 2012)
Longhorn Beetles of Britain Heeney, Potts & Lewington (FSC WildID Fold-out chart 2018)
Websites
UK Beetles website run by Watford Coleopterists Group has a huge range of photos and information about species. There are indexes to search for species by scientific or common name, as well as a classification structure of super-families; a glossary of the naming of beetle parts used in many keys.
Google Lens: Go to google.com and there is a camera icon at the right of the search bar. Click on this, and you can drag and drop any image and it will do its best to identify it – so can be used for any flora and fauna, vertebrates and invertebrates etc. (as well as buildings, landscapes – anything, in fact.) Worth a look, but treat identifications with caution!
Apps