The best book in German is probably "Die Fahnen und Standarten der Armee des Konigreichs Hannover" by Dr Ottfried Neubecker. I have a photocopy of the section on the Napoleonic era.
There is probably the best description in English of them in Keith Over's book "Flags and Standards of the Napoleonic Wars", published by Bivouac Books in 1976. I have this book.
Apologies if you already know this but flags are described by the obverse (pole on left as you look at it) and reverse (pole on right as you look at it). In his book Keith gives details of the following:
Feldbataillon Hoya. Black, obverse white upright sword superimposesd over white oak wreath, reverse iron cross with inscription 2Mit Gott furs Vaterland". There is an illustration of the reverse.
Keith does not mention it but I know that this flag is hanging in the hall of Schloss Marienburg (East of Hannover on the River Leine) because I have seen it there. Schloss Marienburg was the Hannoverian Royal summer residence. It also has a number of KGL infantry colours hanging in the hall and a KGL cavalry standard on the wall.
Feldbataillon Calenberg. White with oak and laurel leave wreaths on both sides. One with inscripion "Zieht aus zum eden-kampf" and the other "Kehrt heimmit seig gekronenet". There is no illustration.
An unknown Hannoverian flag described by Keith as white with on the obverse with a white horse of Hannover on a red circular field surrounded by the inscription "Quo fas et gloria ducunt". On the reverse a white central circle bering a crowned GR III cypher surrounded by a red embroidered border. There is an illustration of this. I have read elsewhere that this does not actually exist but was the pattern for all of the regular flags. If so then they may have been in facing colours. I think battalions may originally have had a variety of flags as they were formd in 1813 but this new pattern was supposed to supercede them. It is not clear of the extent to which this happened.
Landwehr Bataillon Gifhorn. Described by Keith as yellow with an oak tree in front of which is a leaping horse of Hannover in silver, below this a partial inscription ending in "Gifhorn", speculatively this was "Landwehr Battaillon Gifhorn". Keith does not give an illustration but there is one in Dr Neubecker's book showing the reverse and the horse unusually facing to the left. The illustration shows the actual flag, which is missing its lower left corner and is in a very poor state. I read somewhere that this was because of being shot through at Waterloo. I believe that this flag is in the museum in Gifhorn.
Dr Neubecker mentions two flags given to Feldbataillon Lauenberg in April 1813. There is no illustration but his source is an anonymous pamphlet "Das Feldbataillon Lauenberg" published in Einbeck in 1863. He also mentions that Feldbataillon Bremen was given a flag by the town, again no description.
"Nec Aspera Terrent" by Friedrich Schirmer (the history of the Hannoverian army) contains an illustration of a small flag of the Jager Corps. This still exists and is in the hall of Schloss Marienburg where I have seen it. It is dark green with a bugle horn in the centre, "1813" above that and "Kielmansegge'sches Feldjager Corps" below.
You may come across an illustration of the flag of the Osnabruck Landwehr Batallion. This does exist in the Osnabruck museum where I have seen it but it was presented after the war by the grateful inhabitants.
Hope this helps.