B5 paper sheets don't seem to be easily available in the UK.
B5 books are the most common size though, most of my school exercise books were B5 or B4 — that allowed me to glue in a sheet of A-sized paper, possibly folded in half.
To answer your question about my preference... For non-fiction books, A4 pages seem too big for me, A5 pages seem little too small to me, and B5 pages seem just right. Again, this is just for non-fiction books. Fiction books I prefer more A5-sized. Just my own personal, subjective preference. That's all.
B5 books are the most common size though, most of my school exercise books were B5 or B4 — that allowed me to glue in a sheet of A-sized paper, possibly folded in half.
Why do you prefer B5 to A4 for paper?