Standard Reference For Charles-I

For halfcrowns, specifically, I suggest The Half- Crowns of Charles I Minted in England, Scotland & Ireland 1625-1649 - see http://www.spinkbooks.com/index.php...34&limit=100

There are 5 volumes, so choose the one(s) you need.

PappyB and Pertinax
Thank you... I've seen coins attributed with Bull numbers... do you no this reference?
Also, I'm having a hard time understanding Groups and Types and was wondering which references are well documented with images?

This is a basic but great site for identifying all the mint marks for everyone from Edward I to Charles II, Link here

http://www.psdetecting.com/Mintmarks.html

Scroll down to the monarchs and click on the one you want.
It's mintmark identification only - no details on the coins.

I think the 'simplest' starting point is the Brooker Collection book (Sylloge of Coins of the British Isles 33 The John G. Brooker Collection Coins of Charles I (1625 - 1649))

It covers a good range of Charles I coinage, is illustrated (in black and white) actual size and lists Spink (and in the case of shillings Sharp) reference numbers. Not cheap, and a little dated (a few further varieties of shillings have come to light since it was published) but the closest thing to an 'all in one' guide.

For detail, the British Numismatic Journal can be helpful. Available digitally here: https://www.britnumsoc.org/index.ph...ications/bnj

The original work on classification post Hawkins was done by Grant Francis. Half crowns in 1917 and shillings 1918. Further work was carried out by Roy Osborne but his paper is quite detailed and I'd try the Francis ones first via the above link.

Groups and types for shilling are well covered (with pictures) in the Sharp paper Pertinax mentioned. This was updated by Holt et al in 2014 here: https://www.britnumsoc.org/publicat...NJ_84_12.pdf Sharp reference numbers are what us shilling enthusiasts use as Spink is too broad for all the various varieties.

Pertinax also mentioned that Maurice Bull wrote five(!) volumes on the half crowns. Vol III covers those of Charles I.

Hope that helps!