Þ & Ð
I've already deliberated on these two letters here, so i'll be breif.
In short, this letter was derived from FUTHARK after scribes needed a way to write /θ/. This letter was destoryed ny the Fr*nch but it lived on in Iceland. ð is also used in the Intenational Phonetic Alphabet.
Æ & Œ
These letters are two sides of the same coin. Æ is an A and an E pushed together, livewise with O and E for Œ. Æsc[æʃ] was used in old english to write the forms of 'a' heard in words like hwæt(what) and þæt(that).It's remnants can still be seen in words like Æther and Æsthetic. Œthel[ɛθl] was used (allegedly) to write long forms of /e:/. It's remnants can be found of words like subpœna and fœtal.
Ȝ
Ȝ is a very early letter. Back when the latin alphabet was making it's rounds, there was no way to transcribe [j]. Varoius solutions were employed. In areas that spoke some variety of latin, an i with a tail 'j' was used. Most of these places would later adopt 'y', demoting it of it's vowel status. Most germanic speaking areas would go on to adopt j once it was promoted to a staus as a proper letter, however this would not be the case for awhile. Many germanic places opted to have 'g' play double duty it would sound /g/ as well as /j/. You can see traces of this in words like German Richtig[ʀɪɕtɪɕ]. On the english continent, clever scribes altered the writing order of g and produced Ȝ. This was how it was written on the english island, and then the butcher fr*nch happened and now it is no more.
Ƿ
In english there were, and are, two Ws in spoken english. This is beat explained by the differnece between 'Which' and 'Witch'. 'Which' in older english uses /ʍ/, a voiceless labiovelar approximant. This was written using Ƿ. Once again, this letter was lost because the fr*nch, bladebladebla, you heard this story alot now, it's a bit of a running theme at this point. Also, w as we knew it also doesn't exist yet, merely existing as 2 u's 'uu'.
ſ
this is long s. It's s, with a lot more rules attached. It was rather clunky so it faded from use. It lives on thorugh the German 'Scharfes S' (ß).
Ŋ
eng. This letter was penned by a spelling reformermer to disambingulate ng, which can be pronounced either [ŋ] or [ŋg]. This letter never really caught on, though it is still used in some romanisatons of select langauges, as well as appearing in the Intenational Phonetic Alphabet.