I came, I sat, I read - and marvelled

An economics don once assured me that his college fellowship would never have sanctioned its famous Wren Library if it had understood at the time how to ascertain its real cost. It is now similarly a ritual at the opening of every major public building in Britain, as witnessed most recently at Holyrood, to begrudge us the calibre of civic architecture most countries take for granted. It is therefore heartening, six years after the British Library by Sir Colin St John Wilson and Partners received its baptism, to find there is already a lay audience sufficiently appreciative to merit a book explaining its architectural intentions.

After Brian Lang's concise history of the institution, the two photographic surveys and five essays open straightforwardly enough, with Roger Stonehouse marching us from Euston Road to a reading desk illustrated by the first series of Gerhard Stromberg's plates. Ubiquitous as a means of describing buildings, it seems an odd convention to pick here for it can take one only so far with a work that, unusually, reserves much of its delight for those sitting many hours in the same place; a considerable feat altogether harder to pin down.

Something of this achievement is captured in "Affirmation", the second suite of photographs that stands independent of the text. The reading room pictures convey distinct reminders of the top-lit halls by Sir John Soane at the Bank of England captured by Frank Yerbury just before their destruction. But where Yerbury's style of rich contrast exaggerates depth, Stromberg's technique is one of soft, even tones that flattens perspective.

The third essay provides a narrative history of what Wilson ruefully terms the "Thirty Years War". It begins by describing the decade of preliminary skirmishing over two schemes opposite the British Museum, whence the British Library was driven by the conservation movement. Then it relates, with admirable clarity, the extraordinary endeavour over the next quarter century through which this building of prodigious size and functional complexity was brought to fruition at St Pancras.

If expending half a professional lifetime to realise a single work is now less common, there is ample historical precedent for what remained routine into the late 19th century. It has certain consequences, and stylistic evolution over the course of the design can be traced at the library just as at St Paul's. From the outset, however, this book betrays an underlying anxiety about the critical reception to which a building so long in the making is subject. This is strange, for the precise state of architectural discourse at the moment of completion seems inconsequential for a work intended to last well into three centuries.

Wilson's response was to try to put the building beyond history, and the second piece, "Composition and context", contains a synopsis of the collection of essays he wryly calls "Despatches from the front" through which he justified the final outcome. Appealing to the tradition of criticism that holds our species innately predisposed to favour certain "symbolic" forms is honourable enough, but while Wilson's reflections on evolutionary biology and developmental psychology clearly influenced his design, they remain articles of faith in relation to how the building will be perceived in the future.

Such speculations go only so far in crystallising the actual elements, such as columns, windows and doors, through which any architectural composition has to be realised. Quotation abounds in the library and, if many sources are readily acknowledged, the building is presented primarily as the fusion of two formal traditions: those of inventive High Victorians like Butterfield and Waterhouse, and northern European Modernists, such as Aalto, Haring and Scharoun. Each contributes technique for arranging complex functions on irregular sites and direct formal precedents. What is not revealed in the book is the criteria for selecting the latter.

This is not so much an issue in the interior, where "An intimate monumentality" demonstrates the architects' systematic differentiation of elements such as columns, gridded screens and even handrails. Stonehouse does this with a lucidity rare in the over-intricate analysis of the architect Peter Eisenman, who pioneered this approach.

For the exterior, conservation issues determined that deference both in terms of massing and materials should be paid to the adjoining Midland Hotel. In this respect the northern Europeans offer least help in articulating the carapace drawn over the substantial volumes within that had to be largely unfenestrated to protect the contents. Yet deprived of detail equivalent to the mouldings, string courses and arches that originally regulated its effects, the use of Butterfield's strident polychromy proves overbearing on the flanking elevations and only on the northern elevation was a balance reached by vigorous modelling.

If the highest accolades of architectural historians, not just journalists, have traditionally been reserved for those whose formal invention surpasses that of their milieu, this is because time erodes their ability to recover the exact circumstances and original intentions attending the creation of buildings, so that this is all there is to fall back on. Stonehouse attempts no critical assessment of whether the British Library succeeds in going beyond its numerous sources to forge something transcendent, but perhaps it is justly exempt; for while this book remains lodged in its basements there will be a sure and infallible guide to its intended meaning.

Michael Evans is a director of the architectural practice MacCormac Jamieson Prichard.