Gareth King deserves the credit for recommending it
Just read this post in the Forum summary - thank you. Actually, the book can be downloaded as PDF (its 1989 reprint, to be precise): http://digi20.digitale-sammlungen.de/de/fs1/object/display/bsb00052892_00001.html.
Also available online is the even more elaborate 1909 book by Pedersen, to which the “concise” (440 pages!) grammar claims to be an introduction (alas it’s in German): https://ia800306.us.archive.org/6/items/vergleichendegra00pede/vergleichendegra00pede_bw.pdf
The scans are perfectly legal, since both books are out of copyright by now.