The pocket-sized Observer's Books series by Frederick Warne are perfect for collecting due to the uniform size and format.
Covering a wide range of topics, a collection can be formed on one subject, or completists may wish to have every title, from 1-100!
Subjects include nature, geology, music and architecture, as well as automobiles and aircraft.

Over 400 flags and badges of all nations, in full colour.
The Observer's Book #29
ISBN0723200882
VG- hardback in VG price clipped dust jacket. There are a few minor pen notations.
$10 A/B
Frederick Warne 1971 revised edition, 207pp + index.
[#3289 ]

Complete introduction to fungi by a past President of the British Mycological Society
The Observer's Book #19
ISBN0723215650
VG hardback with NO dust jacket.
$10 A/A
Frederick Warne 1977 first edition, 185pp with 64 colour illustrations and 52 black and white photographs.
[#3193 ]

First-hand coverage of every major US spaceflight up to time of publishing.
The Observer's Book #52
ISBN0723215227
VG hardback in VG price clipped dust jacket, sunned to spine.
$20 A/A
Frederick Warne 1974 first edition, 191pp with 14 colour and over 50 black and white illustrations.
[#4562 ]