As a member of the RIBA for almost 30 years I have been visiting HQ regularly every 5 years during that time during my studies and in my professional life.
The Library is a superb space in a superb tour de force of architectural splendour.
The staff are extremely patient, the collections comprehensive and detailed and compared to the www for information it wins hands down.
The problem with the Web is most of what is offered has been put there to win commercial benefit. The architectural library is put there to benefit mankind, there's a significant difference in the two approaches that will be lost on most people but high quality usually is.
Go. You will not be disappointed if you wish to read a book, view images or read some periodicals in quiet relaxed atmosphere.
Diodoros
Google
I visited the library for a research. The people at the library's front desk were exceptional. Kind, welcoming and immensely helpful. They helped me find all the information I needed. Very very pleased!
The building is also very interesting.
Holden Madagame
Google
Lovely library, beautiful and well organized collection, kind staff.