Can you think of a treatise that is easily obtainable - afaik Siksasamuccaya wasn't reprinted and is impossible to buy - and readable? I mean, the Bodhicaryavatara is good but too terse, not really explaining things.
Well, Gampopa's Jewel Ornament
I've heard that Lozang Jamspal (Columbia University) has made or is in the process of making a new translation of the Śikṣāsamuccaya. Much needed and long, long overdue. The old 1922 translation of the Śikṣāsamuccaya is available in PDF here
There is also Richard Mahoney's Of the Progresse of the Bodhisattva: The Bodhisattvamārga in the Śikṣāsamuccaya
. It's a good translation and survey of the root stanzas of the text. It can be downloaded as a PDF once you register as a member (free).
As for the Bodhicaryavatāra, The Nectar of Manjushri's Speech
is an excellent commentary by Kunzang Pelden, who was a student of Patrul Rinpoche. Very traditional Mahāyāna.
And as Will mentioned, Nāgārjuna's Letter to a Friend
is very good. As is his Precious Garland
All the best,