A while back I recall Richard Clarke of TooFatLardies when talking about starting a new period certainly endorsed picking up some ospreys as a starting point and then checking more into whichever period it is if of interest after the intital review.

I have a few tricks for my google shui:

1/ Check Osprey for a title in the period, which normally shows other linked titles.

1a/ Go to Amazon enter that title and look up the entry, you can often get them still brand new but cheaper than direct from Osprey and it will show a here’s what other people bought who got this title, most of the time its linked and could be of help. Amazon entires also get reviewed to help other buyers decide if this or sometimes a better title is referenced.

2/ If got a Wargame Mag covering the period with an article, check the suggested reading/reference which normally gets listed at the end.

3/ Is there a sourcebook covering the period/particular camapign such as done by Warlord, this will often point you into other books, Warlord is normally Osprey but other company sourcebooks can cover various publications.

4/ There’s a how to Wargame this period XX guide, various ones are around these days and normally under a £10.

5/ For historics definately check this site:

http://www.plasticsoldierreview.com/Periods.aspx

Its a 1/72 review site, but under whichever box its evaluated there will be a list of were they got there information which can often point to a wider list and often references a Millitary Modeleing or collector magazine that had run a feature and might be worth picking up if can locate a cheap enough version in the second hand market.

6/ Lastly other good sources aside from Osprey although not always with the purdy pictures in there works:

https://www.pen-and-sword.co.uk/

https://www.helion.co.uk/

These are UK, but if across the pond, the plastic soldier and Amazon site for the states can help in research.