Reddit mentions of New Comparative Grammar of Greek and Latin

One very good source is Sihler's A New Comparative Grammar of Latin and Greek.

In addition to telling you a lot about Old Latin, the index lists Old Latin forms.

A good academic library will have this book, but if you're interested in the pre-history of Latin, you may want to buy it--this book rewards careful and repeated reading.

Edit: Looks like a good deal of the index is available to view on Amazon, so you should check it out. NB: The Old Latin forms are marked by "OL."

Edit II: How could I forget Palmer's The Latin Language (1954), which has a great index of archaic texts. And Baldi's The Foundations of Latin (1999), which I haven't read, but it looks good.

These two resources take an approach very similar to Mounce (phonology to morphology). However, they go further back than Mounce to what is known as Proto Indo European in the formation of Greek, Latin, and Sanskrit words. Both resources will not only help you learn Latin, they will also greatly improve your Greek.

https://www.amazon.com/Comparative-Grammar-Greek-Latin-Darling/dp/0226079317/ref=nodl_

https://www.amazon.com/New-Comparative-Grammar-Greek-Latin/dp/0195373367/ref=mp_s_a_1_5?keywords=Greek+Latin+grammar&qid=1566456418&s=gateway&sr=8-5

This is probably the best comparative grammar book I've read with a focus specifically on Latin. It is simple enough for a student of Latin to read without having been completely indoctrinated in reconstructive language study, but is also extensively comprehensive. This is the publisher's website, which says there is an updated edition coming out this summer.

Alternatively, Sihler and Buck are both good sources for answering questions of comparative grammar, and you might be able to more easily find those online through various sources.