It is not easy to answer this in a definite way. I think that including copyright items such as book covers or illustrations might be covered by one or other of two of the exceptions found in sections 29 and 30 (1ZA) of the Copyright Designs and Patents Act, namely copying for private study
or for quotation
. The problem with section 29 is, as you have acknowledged, you would be doing the copying in a commercial setting (ie you are being paid for the completed work). It is also stretching the idea of research and private study because the copying* is not being carried out by the person whose private study it is. Unfortunately another exception for copying the purposes of personal use which would have suited your purpose more closely, was briefly made legal but has since been rescinded due to a legal challenge as to its validity under European law.
The second exception, for quotation, avoids the problem of the commercial aspect, but as this is a new provision we have no guidance on how the courts will interpret 'quotation'; does it refer only to text or can it be used more widely to refer to graphic works?
Both these exceptions would require an acknowledgement of the source work "unless this would be impossible for reasons of practicality or otherwise". I suggest that proviso could be arguable in your case, or alternatively, perhaps the requirement could be met by a small acknowledgement placed at the back to the album.
Ordinarily I would suggest that the uncertainty over whether one of these provisions might be of assistance means that you should not go ahead. However, these albums are for the personal use of your clients and so it is highly unlikely that the owners of the copyright works would ever become aware of the inclusion of the images. In other words there might, theoretically, be grounds for a claim of infringement, but in reality this is highly unlikely to arise.
Perhaps the best approach would be to ask the client to provide the actual image which you then incorporate into the album, thus placing the liability on them. Clearly this might not be acceptable if you wish to control the quality of the items you incorporate, but assuming that the client owns the book etc from which the image they wish to have included is taken, this would fall more squarely within the private study exception since you would merely be collating the item and not copying it.
* Section 42A of the CDPA does allow the copying of a work for private purposes to be done by a librarian or archivist, but section 43A makes it clear that this provision is strictly limited to these sorts of individual, and in addition, the copying must be supported by a specific declaration signed by the person requesting the copies.