There's a Guide Association book from 1990 ish called "1910 and then..." that I found really interesting when I was looking at doing the Commonwealth award. It's long out of print but turns up occasionally quite cheaply on ebay. It covers the start of guiding and then for each of the eras that follows it describes the main happenings and changes with black and white pictures and detailed timelines, although of course it doesn't cover anything post-1990! A quick look on ebay turned up this copy: 1910...And Then? A Brief History Of The Girl Guides Association Booklet 1994
Girlguiding published a book called "Something for the Girls" to celebrate 100 years of guiding in 2010. It's more of a celebration book than a history reference but it does contain lots of really interesting snippets about various aspects of guiding's history as well as hundreds of photographs. (This book: Something for the Girls, Alison Maloney, Hardcover, New)
(I'm in no way connected to either ebay seller, just included the links so you can see the books I'm referring to!)
Try asking around local leaders and units in case they have either of the above or similar books sitting around that you could borrow - older units especially may have all kinds of resources hiding in their cupboards, not just history books but also old handbooks or log books!
Also, one of the other leaders on this forum, Fenris, has a really interesting guiding history website that's well worth a visit too: Leslie's Guiding History Site
There are several Guiding history books out there (there is a list of several on my history website) - the thing to be aware of is that in terms of how Guiding started, they can be misleading, and in many cases actually inaccurate. I've done quite a bit of research and shared my findings (and those of other people) on my Guiding history website. I regularly make new discoveries too. Also happy to answer any specific questions where I can.