I picked up and bought one Penguin Guide, sometime in the 1990s. Had lots of Glazunov and others in it, at least a little Schmidt, etc. (some of which I picked up on my limited college? grad student? oy, when was that... allowance, and very happily so too- the Schoenberg/Schmidt with Mehta on Decca comes to mind, some others...), and even a work by Holmboe (his concerto for brass instruments) in the collections section (this was before I read many reviews of Holmboe recordings in Fanfare magazine, mostly by Paul Rapoport, dived in with the 8th quartet, and became a fan of that composer.) Oddly like the Oxford Compact Dictionary I bought much more recently, it had some very strange omissions, too, but if the present flightless birdie ... erm, Penguin Guide skims over the lesser-known of the Romantic Era, it's a recent and regrettable phenomenon, they've done much better in the past...
Eric