[Roy Dennis]

When you say "using the available lower book id" are you saying there are lower numbers available ?
When Hoster imports it normally selects the lowest available book id. Once you have used up all three figured book root id's 1 to 999 which gives you your 5 figured book id once the track # is tagged on then it has to add the extra digit.

However if you are downloading single files and importing each separately each will be allocated it's own Book Id which will quickly swallow up any available three figured root book id's. You can overcome this by importing them in multiples of up to 99 at a time and setting the define field order to ignore Disc Id and Track #.

Or what I do which is a bit more time consuming, I rename the downloaded file with my own Disc ID and increment it's track # by one each time. This allows me to tag new songs onto an existing Book Id up to a total of 99 tracks. When importing this way I set the define field order to recognise my disc id and track # correctly, it does work but you must ensure all the files are named in the same order and get the spacer between the Disc id & track # consistent on all or it will overwrite an existing import.
I tend to create separate folders for each manufacturer as some put name artist - title other's title - artist.